There can be dwarfism in Chihuahuas, we worried with my first tiny girl ( 8 oz at 5 months old when we got her) had dwarfism but it ended up being a liver shunt. My new girl we got almost 2 weeks ago is 11 oz at almost 3.5 months, her conformation does not look correct at all, not much of a neck and very short back. We are watching her grow to see if dwarfism could be an issue. The bad thing about dwarfism is with a tiny body there can be smaller more compacted organs or other things that did not develop quite right which could lessen their life.
With your baby only being 9 weeks old things can change a lot as she will grow a lot. Gidget as said above was 5 months old and 8 oz, she is now 2 lbs 3 oz (following the charts she would have been around 1 to 1.5 lbs as an adult). My long coat boy I just lost at 11 yrs old was 1 lb 4 oz when we got him at 4 months old and ended up 3.5 lbs but towards the end was around 4 lbs. Runts can outgrow littermates, and vice versa. A lot plays in to what a Chihuahua will be when grown, parents size, grand parents size, etc... I find that a lot of chihuahuas follow the charts up until around 4 to 6 months and then can have major growth spurts and shock you. And no matter how trusted a breeder is, they can not guarantee size. They can give you their best guess. I am eager to see how my new girl grows and praying it is not dwarfism or any other major defect causing her size but there is something usually underlying when they are this small