There'd have to be a pretty huge size differential for it to be anything other than mildly uncomfortable. I have the same problem, though it's not that bad. The wedding ring moves around a bit sometimes but I'm used to it now. Getting used to it on the other hand took quite a while seeing as I never wore a ring before.
As for fattening up the fingers, take up guitar. I'm 4" taller than my dad and about 50 lbs. heavier at the moment yet his fingers have always been thicker than mine, even though mine are longer. Either he just grew that way or it's the years of guitar. Worst case scenario, your fingers will stay the same size, but you can impress chicks with some emo crooning.