http://lists.libsdl.org/pipermail/sdl-libsdl.org/2008-July/066064.html
This is kind of creepy, but I am having exactly the same problem , and
I went about testing for it in exactly the same way. That is to say,
if I press a keyboard key in my program, a function is called that
does screen = SDL_SetVideoMode(1000, 1000, 16, SDL_OpenGL), (and then
I reinitialize OpenGL attributes appropriately), and my activity
monitor claims my program is using up much more RAM (proportional to
the size of the screen resize).
The reply to that post may be correct, and solve my problem, but I can
resize the screen hundreds of times until the computer starts to get
very sluggish. Maybe I should try resizing and then waiting a while
(of the order of minutes) to see if my OS (OSX, like the guy who
posted that) reclaims the memory? (I’ll do that when I get back to my
computer), but for now, any other ideas?