When you say big, do you mean bulky? Or too big (as in gaps around the legs)? If it's just bulky, then yes, cloth *is* generally going to be more bulky than sposies. I've never minded personally - I love a nice, squashy, cloth bum. Sposies look unnaturally thin to me!
If you can't get his vest done up, you can always size up or use a vest extender. It's not a good idea to have a tight vest on over a cloth nappy as it can cause wicking.
If there are gaps around the legs and tummy, however, then it might just be too big for him. My girls were 6lbs ish at birth and I had to use xs and newborn nappies for quite a while.