Because of the sheer amount of frenchies bred nowadays (and not all too many of them to standard) they are going to vary wildly in size.
Just like when staffies were the 'in' thing, you could get tiny ones, ones that fit the standard, or massive 30kg hulks.
Even well bred dogs can produce oversized or undersized pups though.

Assuming he's not overweight and stays that way, he's just a big boy and there's nothing you can do about it really, even if he winds up about 15 - 20kg
As long as he's healthy (well, as healthy as a frenchie can be), don't worry about his size and just enjoy him as a pet

 

Well so far he has been really healthy hopefully he stays that way.
Its just when you look up the standard and he is above it already and people start commenting on his size you start wondering how big is he is going to get.
He is absolutely adored by all of us and a little bit spoilt.

 

I know a good few larger frenchies and tbh they are much better put together in terms of conformation than the smaller ones. Their eyes are less buggy, they have a longer muzzle, longer legs, and are generally healthier in terms of breathing issues.

 

My Suzie was quite big for a Cavalier.
I've seen much smaller ones around. She lived to 15 so something must've been right.