I have a bit of an extended family as well - I have 1 biological sister, and I definitely want her there. But then I have 3 half siblings, and 4 step siblings. And all of them have children. I wasn't raised with any of them, so it's a different relationship. I love them all - we're just not super close. I did end up inviting all of them, and honestly, it was mainly because I knew most of them, if not all, wouldn't come. If I had thought they'd all come, I am not sure I would have invited them. But at the same time, I keep reminding myself that it's really not that much extra $$ for me when more people come, the financial burden is mostly on them.
I think most people probably know that they're not super close to you, I can't imagine it would be too much of a surprise. There are many relatives that I wouldn't expect to be invited if they had a DW.
I wouldn't worry about the concern of having more guests then your FI - that's just the nature of families and DWs! I think Paul will have 2x the people there that I will - mainly because his family is bigger, and he has a larger circle of close friends, and none of them have kids, so travel is easier. This is also Paul's first wedding!
Anyway, I really don't think that helped, I was just sharing my situation I guess.