My dog is a 6 moth old rottie and I'm woundering about his chest size. HIs body is growing super fast, but his chest is rather small.. Is it because he has a slight trace of the collie on his mothers side? Or is something wrong?

I would have to say it's got to do with the Collie in him. If you're worried, have the vet give him a once over.

His form is bugging me

You don't like the way your dog looks? Poor dog.