It seems that Qt does not release unused memory

[Brandybuck]

As far as I know this is a normal behaviour. Your kernel prefers to have the block of memory assigned to a given process in case it wants more memory at some later point in time. If you ran out of memory, it'd be freed and assigned to the process being in need of that memory.

This is true for Linux, but other Unix systems have differing memory allocation behavior. Try your application under FreeBSD or Solaris for a comparison.

[wysota]

Yes, of course. That's natural as every system uses a different allocator implementation. The author said he was running Linux. Furthermore one Linux based system may behave differently than another, it depends on the kernel it is running (both version and options).