All text should be using cleartype for proper font size scaling... but if you're trying to resize a smaller-point font within the scene, you're going to be limited to the font size you set (it's resizing a rastered image at that point, so it's just blowing up a low-resolution image).
General rule -- set a high font size that's as large or larger than you need in the scene, then resize it in the scene to fit where you want it.