this is EXACTLY how mine will be. i don't have a huge extended family, only a couple aunts and uncles who will be in attendance - no cousins will be able to make it out. i also don't have many friends. but he's got an enormous family, and they're so excited and every one of them will be coming, and he also has a bunch of friends he's inviting. so the guest list is pretty lopsided, but that's just the way things are. there's no reason to make him cut back his guests just because i don't have as big a family or as many friends as he does... they're important people in his life, and they want to celebrate with us.
besides, you'll get to meet everyone that matters in his life, and he'll be thrilled with it.
the only concern i'm having is how my parents and i are paying for the reception food with that many guests, but we'll make it work. we may even be splitting the cost somewhat with his family, because most of the guests are his. you'll have to decide how to handle this part, but i'm sure you can reconcile how things are working on the financial side of the plans.
good luck!