i got 2 stimmies from the same clutch- they eat the same amounts on the same days, but one of them has always been bigger (the male), theyre 10months, and he is just over 60cm, and the female is only about 50cm (the breeder i got them from said the female didnt feed when she was young- she refused 3 feeds and then was forcefed)- and ive heard that if theyre slow to start, they miss out on their first growth spurt... and it takes a few months for them to catch up.
Sometimes breeders feed them minimal amounts to keep them small (and cute i supose) in order to sell them.. so ive been told. I think the reputable breeders in WA are all very good.. but you never know, where did you buy yours from?
Another thing.. some snakes are just small. I'm wondering if my female will be a small snake. Comparing her with her brother, who is huge, it just seems likely that she wont grow that big in the end.
I wouldnt worry. Just keep finding your stimmy, and as long as he wolfs down his food, he'll catch up. I feed mine once every 6 days- they get 2 rat pinkies each.