"Oh, look! He thinks he's one of the big dogs!"
"My dog doesn't know how big he is, he's always trying to sit on me!"
These are just a couple of phrases that dog owners say that really miss the point of their dog's behavior. Owning 3 dogs of widely different sizes has taught me many things, but one in particular: size does matter.
Training a larger dog is easier than training a smaller dog. Larger dogs are more adjusted to the size of our world, and there are only a few situations in the household that pose an "every day danger" to a larger dog because they are more durable and less fragile. Some larger dogs also possess strong "working" instincts (chasing, herding, shepherding, tracking) which can be learned, honed and fulfilled. Some smaller dogs have 0 "working" instincts and mostly react to their world with the baser instincts (forage, play/bonding/sex, territory and self preservation.) Self preservation is the big one, here... because little dogs KNOW they're little. Or at least, they should be taught that they are. There are certain things that big dogs can do that little dogs JUST CANNOT, and it's a danger to them if they're encouraged to act beyond their limits.
Often times owners will put their smaller dogs in a situation that is "dangerous" and not know it, in turn making the little dog react in ways that aren't just because "he thinks he's so tough." Sometimes, a smaller dog can be allowed to play too excitedly, putting him in danger for a "play accident" with a larger or less playful dog (a miscommunication.) Miscommunications happen just as often with dogs as with people, and finding the right (and SAFE) situations to socialize a small dog are few and far between. A lot of dog owners are blind to their dog's overbearing, aggressive or disobedient nature, and sadly to say, there are a lot of those dog owners at a dog park. A "correction" between dogs is completely natural (one dog snapping or "jumping" another dog in cases of extreme social rudeness, i.e. humping, pushing or incessant barking) but a correction from a larger dog to a smaller dog, even if meant well, can seriously injure a small dog. This is why it is difficult to socialize a small dog: You must find other like-sized and easy going temperamented dogs to be patient with your little dog and, above all, you must keep your little dog away from dogs that have prey drive. Prey drive in larger dogs is a serious threat to their smaller counterparts. Frighteningly, some owners don't understand or even know their dog's prey drive, and through no fault of anyone's (really) a larger dog can end up attacking a smaller dog out of curiosity or extreme excitement. I've had a few such instances happen to my chihuahua. These are difficult to reverse, but avoided by staying aversed to "over excitable" and "disrespectful" dogs.
Speaking from personal experience, the socialization of your dog is most important above all else. Socialization, in my mind, is a set "toolbox" of interaction defaults in which a dog can comfortably and clearly navigate the situations he will be placed in. For instance, I teach all my dogs to "shake," because when they meet strangers "everyone shakes hands when they meet for the first time." Informing the stranger of my dog's "manners" and asking them to "please shake my dog's paw first" not only sets boundaries to the stranger right off (telling them what my dog is comfortable with so my dog knows I'm in control and setting boundaries, seeding trust to all parties) but it impresses them and calms both parties if they are, somewhere on the inside, afraid of eachother for any reason. Teaching "shake" is a great socialization tool because a human will always react positively to this, rewarding your dog with a smile or a pet for reasons that your dog understands, "Oh, this person wants to pet me because I did a good "shake!"" thus relieving the social pressure that can be on a dog when strangers just reach out to them, stare at them or wish to give affection for "no reason" (which can confuse and scare some shy dogs.) Little dogs have a tendency to be shy. Why wouldn't they be? Everything around them is a million times bigger and louder than they are, I'd be shy, too! Teaching your dogs tricks like this are TANTAMOUNT to their courage, self esteem and coping ability.
Little dogs are very sensitive to their environment because it's evolutionarily practical for them to be so. When you're very small, all you can be sometimes is an alarm. This is the reason they have a reputation for being aggressive and "yappy." A little dog has the bursting heart of every dog in their little breast, but there are some instances in which stepping in for your little dog will help him gain trust in not only you, but "new situations." When a dog can accept "new situations" without fear or withdrawl, that is when he is most equipped to live and learn with you in our crazy human world, and default to your judgement. It is important that a dog has a "default" behavior in their socialization tool box. My dog's default behavior is a "sit." Whenever my dogs are confused or need help, they always sit, or come next to me and sit by me, letting me know, "Could I please have some direction?" Asking for direction is also an instinct that all domesticated dogs have, as proven by the many studies comparing them with wolves (their closest relatives.) This default behavior is usually chosen by the dog themselves, a tried and true "trick" they can ALWAYS do to be successful in a situation. It's always helpful to have this default behavior be a "sit" or a "lay down." Other behaviors may be too distracting.
Aggression is a hard-wired coping mechanism in dogs, but it is NOT their first go-to. Dogs are social creatures and try to avoid conflict by nature, unless they figure out that the situation is more quickly remedied (the "interaction" or "negative stimuli" stops) when they use their teeth and mouth. A little dog figures this out when a human does not step in for them, and as we know, there are SO MANY MORE situations in a little dog's life in which they need our help and guidance. When a little dog is forced to cope in a situation he'd really rather not (examples: Being asked to interact with a larger more boisterous dog, being asked to be around fast moving and unpredicatble children, ect.,) after using all his "avoidance techniques" (which a lot of beings ignore sometimes) he will use his "Hail Mary." He will become aggressive and lash out or extremely fearful and run away. Unless he has a good social toolbox and a default behavior to rely on, he will make up his own... and it's usually effective, but not very nice.
Owning a little dog can change you. Where you were once carefree and comfortable with most situations (particularly with a larger dog) owning a little dog makes you a bit more nervous and worrisome. Partially because the little dog stole your heart and you realize how tiny and fragile it is, but partially because you are uneducated and don't know which situations, people, places and dogs are best for your little dog to be around. That's not your fault. There aren't many. Working in the training/grooming industry for 12 years I can tell you that it's very difficult to find "just the right conditions" for your little dog to learn, thrive and feel fulfilled without there being dangers. Dangers like cars, prey-drive dogs, unsocialized dogs & people... even falling laundry baskets, jumping off the couch, doors and just plain accidents that a larger dog could brush off, but a smaller dog would end up in the ICU.
So, size matters. It is a known fact that before you react you should "Put yourself in their shoes." This is particularly important in helping your little dog's personality shine through while keeping them safe from harm.
BONUS: PONYO AND BEAR, PERFECT SOCIAL COPING.