This is usually caused by a memory leak. When programs run, they request additional memory. Sometimes, (usually due to a hard-to-find bug) they don't release this memory when they're done with it.
In the case of SWTOR, every new model/texture has to use memory to be displayed. If there's a memory leak that occurs every time a model is loaded/unloaded, eventually, you will have a lot "leaked" memory that is "used" by the game, but isn't doing anything.
So if it typically uses 2 gig, and 1.7 gig is lost, you get your problem.
The solution is to close and relaunch the game, because Windows reclaims your memory when the process ends. Sometimes, you will require a reboot of windows, if there's a memory leak in your drivers, or in Windows itself.