If you're unable to run instruments, how are you determining that figure?
There's no fast way to tell what's what. You'd need to dump out the instruments data for me to look, but I'm still quite confident that it's a combination of atlases and sounds (if not set to music, will be stored entirely in memory). Short of including things like video, there's nothing that eats up that much memory.
It may also help to monitor what happens when you purportedly unload things - does the memory usage drop?