Not so weird. It's "saving" previous changes.
Redo the experiment, up-to some point, then start doing Edit > Undos and watch the memory usage start ticking backward.
Now, that said, I've noticed that iPhoto is bad at releasing memory, so wild guess here, Preview is sharing some of the same code for "Adjust Color..." and will be just as bad in not cleaning up after itself. So, will free up some of the used memory, probably won't get it all back. The more you work on, more memory will not be freed, so, good idea to maybe every now and then to quit/restart Preview.