[Crayolalaura]

I've had this very experience myself
My mum and I bought a hamster each from the same litter. The one I chose was the largest of all of them and ended up looking like rat when fully grown LOL (I exaggerate there lol)
I took him to the vet and they too were a bit shocked. I gave them an account of his eating habits and activities and they were a bit stumped and just said giving the healthy food and high activity there's no reason why he'd be this big but because he was the biggest that perhaps this was just his natural size
He died aged 2 years 9 months from old age- no problems.
Sorry that's not much help to you lol! But thought I'd share my experience of having a gigantic hammy lol!