Thanks. I was probably misremembering Tim Bunce's YAPC talk: https://www.youtube.com/watch?v=GIIeOntmojg. I'll have to review it again when I have a moment. Your research looks thorough, though.
Update: At about 4:10 Tim discusses malloc behavior, and suggests that larger allocations are mmapped, and the gist seems to be that while rare for malloc to actually give back, it is more likely for those larger mmapped chunks. So, thanks for motivating me to find that talk again. I'll have to watch the rest of it again this evening. It does confirm your assertion, too.