> 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 you don't need for the lifetime of that 1000th object (better hope it's not long lived!).
Thanks to the wonders of virtual memory, you can madvise(MADV_DONTNEED), and return the memory to the OS, without giving up the address space.
Not giving up the address space feels like an anti feature. This would mean, among other things, that access to the DONTNEED memory is no longer a segfault but garbage values instead, which is not ideal.