I'm using a GeForce GTX 580 with driver 327.23, and I've noticed that some of my textures use a lot of memory !
After some GPU-Z memory monitoring, it seems that all my textures allocations are rounded-up to the next POT size.
For example, I allocate a 1024*1025*RGBA32F texture using glTextureStorage2D(). (no mipmap, no anisotropic filtering)
It should takes 1024*1025*4*4 = 16793600 octets = 16.015 Mo
But according to GPU-Z, it takes 32 Mo ! (next POT after 16)
I've done some tests with various sizes, and it seems to always be the case... is it the expected behaviour or did I missed something ?