I think it can depend on a few things, I know for sure that the type of keet will make a difference, my bird Frodo is a reccessive pied and although she is a long bird she is very skinny looking, and most reccessive pied I have seen are similar, especially compared to birds that dominant pied or clearflights, Ricks bird Gebo looks like a nice plump bird and when I looked up clearflights they tend to look that way, very bigger bird, I think Rick or someone said they are almost a mix of a regular budgie and an English budgie, making a bigger bird. Also they don't grow all that fast, from a tiny baby they might but probably after 6 months they don't seem to grow very fast.
So point is your birds are fine, if they seem healthy and eat well then I'm sure they are perfectly fine. Well good luck and if it is bugging you still you should just take them to the vet where they will weigh them for you and tell you if they are in a good weight range. It never hurts to have them checked. Let us know what you do but I think they are fine. Jloc