Appa is small?

I posted last night in a FB forum about Appa only being 280g at almost 4 months old (April 19th is his birthday). They're trying to say it's because his cage is too big and I let him out, so it's inhibiting his size...which I'm sort of not buying. Chinchillas in the wild aren't inhibited on play, and I feel like he can do more IN his cage than out of it. Most of the stuff I've read (or can find) says that letting a kit out for playtime is FINE, but all these people and breeders are saying that it's no better than a deathball, that he's going to stroke out, over heat, etc, etc. The size thing, I can kind of see, but even still...as long as he's consistently gaining weight, whether he's on the small side or not, why is it such a big deal? Are there any studies showing that kits let out for x amount of time are significantly smaller? Or that it will for sure, without a doubt, cause death from heat stroke/heart attack/seizure/etc?



I hate conflicting info, and want to clear this up.