I'm guessing it's size of function rooms. Especially in pubs. If a pub has a bar/room where they can host functions rented by less than the capacity of that room then realistically, without a large bar fee they're losing money over just opening that room. Especially if they're rammed, something nice pubs can almost guarantee on any given weekend. Others may have a smaller function room than we'd require meaning that they'd have to close the venue for us in order to accommodate. As 40 or so patrons will not spend as much as the overall takings of a full evening or even a partial evening closure could cost, they'd rather tell us we're too big a party to host than unfairly set an expectation on how much alcohol we need to pay for that we'd never be able to drink (unless we were thinking champagne all round haha).