The button text should get bigger if you increase the font size (obviously) but it doesn't always make sense to also scale the button's padding or shadow effect with the text, for example. As the text gets larger, most designs have to sacrifice space to make it work. Often this can be accomplished by fixing the size of pretty pixels that don't take away from usability.

It's only a way to help ensure the design scales. Otherwise it would be no different than zoom, as others have said.