Check the font setting at Document Properties - Dimensions - Linear, since the settings for individual dimension types will override general dimension settings. I don't have a lot of confidence in that, but it's all I can think of.
See Text Scale in this help topic:
My guess is you have Always display text at the same size unchecked. It is checked by default in SOLIDWORKS document templates, so if this is the issue, then someone must have unchecked if for this document. With this option checked, the font is always shown at the same size on the screen regardless of the model zoom scale. With it unchecked, the size is shown relative to the model size and therefore changes as you zoom in/out.
I hope this helps,