I think it depends on how good communication in the family is (and how large it is) in general. My mother is hooked in to the family gossip much, much more than I am; I know who nearly all the first cousins are and have reasonably good relationships with most of them where we speak at least a couple times a year, but I live several states away from the town where everybody else still is and could not tell you what relationships they all have to each other, and whether Aunt L would be super offended if I invited Uncle J's ex-stepkids but not Cousin K's whole family, or whether inviting Cousin R means that his sister definitely won't come. Or whatever. I might or might not ever directly be effected by the ensuing grudges and griping, living several states away, but I would still rather not cause more than I have to!

Basically if I was planning a wedding I absolutely would ask my mom who she thinks I would be expected to invite at any given wedding size, early in the planning, and also how many of them would expect an invite but definitely not come (and tbh she would probably have to ask her sister for help) and I'd be glad for the offer.