Is the restriction on the amount of liquid in the container, or the size of the container itself?

The 3-1-1 page says:

Liquids, gels, aerosols, creams and pastes must be 3.4 ounces (100ml) or less per container

It sounds like it's on the amount of liquid, rather than the container itself.

I ask because I have a nice 4oz Nalgene travel container. What if I filled it half way up? That way, it's clearly less than 3oz, probably closer to 2oz.