Failed usage of GL_COMPRESSED_RGB

[Kopelrativ]

I did a simple test, and replaced GL_RGB with GL_COMPRESSED_RGB, and GL_RGBA8 with GL_COMPRESSED_RGBA (OpenGL 4.2).

The result was that GPU memory consumption increased! I know the result will not be the same on all targets, but I didn't expect that. Using GL_RGB5, however, improved GPU memory use as well as rendering speed.

Are there any guidelines (except doing a test on each target and choose from that)?

Edit: mental arithmetic is an unreliable method. I was wrong, and it was an improvement in both cases. Any views on guidelines are still welcome, though.