> For example, suppose you allocate 1000 page sized objects and then free all but the last one. With an mmap based heap, you can free all 999 other pages back to the OS whereas with sbrk you're stuck with those 999 pages

Actually... you can free those 999 sbrk() pages using munmap() on Linux and Darwin (so most likely the BSDs too). You can also change the mappings within the sbrk()-allocated range, much like any other mmap.

This feature is not well known, nor particularly useful :-)