Some chins are simply smaller than others. We have adults who are 900 grams and adults who are 500 grams. Both are healthy, they're just different weights.
You didn't mention if your 10 month old is a standard grey? Because, if he isn't, some colors take longer to reach their full size. For example, I've heard it said that whites are still growing until they're about 2 years old. From personal experience, I've had chins gain some adult weight after 10 months - there really is no "cutoff point" where the chin is "for sure" done growing.
The originating from a pet store probably doesn't, in itself, have much to do with the weight. Chins originating from a petstore are typically from a wholesaler who breeds chins and sells the chins they aren't going to keep, wholesale, to the pet store. Now, of course that wholesaler doesn't want to sell the kits he thinks might do great at show (if he can tell that at that age), so he might sell the smaller ones, or, he might just sell whatever he's got. I've gotten in rescue pet store chins... some are tiny (we had one just topping 350 as a 3 year old), and some are huge. One of our more recent pet store rescues looks like he could definitely place at a show - large boy, nice color, confirmation, density, everything. Part of the reason for the variation is because at the point in time when the wholesaler sells the chins, it's difficult, if not impossible, to determine how a 3-4 month old chin is going to look when it's primed and has its' adult coat in.
Switching the diet may help. We've had chins put on quite some weight and we even had one double his body weight on a better pellet (course, he was on a really bad one to start with and only ate the treats). So it's very possible he may put on some weight. But growing and putting on weight are different things. He may gain weight... but not look any physically bigger.
Some chins are just smaller than others.