I agree with T and Tracy (and nice of you to remember dear Phoebe T haha). I just always say Phoebe's trying to keep weight for her modeling career
She's just built that way. When she eats I can SEE her get bigger, but then she digests/works it off and she's a stick figure again.
I also think the guys that are built like this (or at least Phoebe) has a ribcage that is large for their frame. The tuck in from an overhead view from her ribs to stomach is DRASTIC, but then when you actually look at her ribs compared to the rest of her they just look huge.
I can see/feel parts of her ribs,spine,hips, but she isn't sickly and she doesn't act lethargic or anything that would point to her having an issue with her weight. I'm sure I COULD try to make her fat by force feeding her, but I'd rather a slightly thin dog than an overweight one!
Also after our vet visit I realized I was way off on her weight. I've been saying 3.5-4lbs, she's 3.1 without breakfast haha.
Now Schroeder isn't built the same svelte way that Phoebe is, but he also has a prominent bony chest and large ribs. He is at a good weight, I can easily feel but not see his individual ribs and he has a nice tuck up, but because he's got a big ribcage people always assume he's smaller. So many people are not USED to seeing dogs at a good weight. Even dogs who are not FAT, are still not the ideal body condition and people don't realize it. Dogs are supposed to be trim! Sorry for my rant haha.
If you are seriously concerned with her weight and its nothing medical you can up her food, and some puppy food, canned food, or continue with special chicken or beef added whatever you'd like. You can also look up the recipe for Satin Balls, they seem to put weight on dogs but I've never used them. But she may also just be built this way and always be thin.
(Also Jerry's Mom - I noticed you said you feed Wellness Core and add Royal Canin. It isn't a good idea to mix grain free and grain inclusive kibbles because they digest at different rates. Just a heads up