My experience is that "purge" works the first time, but after that barely if at all and the first time it only gives back a small percentage.  I've seen leakage simply by changing materials on objects.  The first time you realize that the memory is dropping is when you get the "system resources has dropped below........" message  and all you've done is change materials yet object  size has never changed.   This particular leak has unfortunately been around for several  versions.