Yes, that seems to be for textures. Better do something about it, at least use a compressed format before releasing that game.
That's weird, all my wads combined only take up 170mb the textures are all compressed (.dds) format. Maybe it gets uncompressed when the game is loaded?
that's strange because the great advantage when using .dds is that - even with compression - the file is already in the format that direct-x uses which means theres no need to convert it.
so, if you use .dds textures the file size on the (v)ram should be exactly the same as on your hard drive.