I stand corrected. In C too, there is no guarantee memory will be returned to the system and make the program smaller.

From gnu libc free :

"Occasionally, free can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to malloc to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by malloc."

So it seems that it is not just an interpreted language problem.