I have another FMA theory I have while rewatching the show (that someone probably already stated or can answer)… The reason Edward is so small despite his age isn’t from a lack of milk. It’s because of his father’s genes in which Edward physically ages slightly slower than others. Alphonse, who takes more after their mother, ages more like a normal human. 

Thoughts? Better yet, does someone have the answer? 

[https://genetics.thetech.org/ask/ask279 < If you want to learn a bit about genes, I think this is a pretty good explaination]