The OS should make all the memory available after your program exits, despite any memory issues that might exist while your script is running (and it appears you don't have any?).

My experience with OSX is that after a program exits, the OS keeps stuff in memory that "might be used again soon", but all of this memory is actually free to be re-used should any program ask for it. A memory "purge" clears out all this stuff, but purging memory doesn't make a working difference, as far as I see.

As well as the links above, there is a useful discussion here.

I hope that's of some help