We have some evidence that Rosie is an outlier - Bilbo's parents had one kid, Frodo's parents had one kid (though that was limited). According to a glance at the http://lotrproject.com it seems the known hobbit families are large, but it is possible that they're mainly the "gentry" and the amount of single/small families was larger (however, Sam is "lower class" apparently and is the 5th of 6).