Well if they're 7 months old then they aren't fry anymore, they're adults, Lol. Also, 2 inches isn't particularly small for Bettas, it's actually within normal range. Unless you're counting the tail? At that age you only want to count the body when talking size.
Other than that, we'll need more info such as, how were they raised, what size tanks at what ages? What were they fed? How are they housed now? Males are seperated, right? What size tanks are they in?
Bottom line is, at 7 months you can't expect them to grow anymore probably.
Linda
P.S. Edit: I just reread your post and you mention feeding them MW's and BBS...um, you have started feeding them other larger foods, right? BBS and MW's would be WAY to small for adult fish of 2 inches in size. They should be on frozen foods (bloodworms, adult brine shrimp) and possibly live if you are able.