More spacious doesn't have to mean worse: I bet that some bat species prefer to stay with their colony, while others look for a smaller summer residence just for their family. And yes, the more different places in which you put bat houses, the greater the chances of them finding an occupant. Just remember to use only nails and untreated wood; no glue, paint or other chemicals, as bats don't seem to like the smell (I can't say I blame them).
Maybe making a nice marsh with tons of mosquitoes would also lure them
One of the many things I haven't been able to find out is whether bats come back to the same feeding grounds each summer, and more specifically whether they remember where they nested last year.