Different fonts will behave differently. Some fonts resize well, others don't and display weird and unpredictable artifacts when resized. I wish I knew enough about fonts to tell you which types of font resize better. But my point is: What you're seeing is not unexpected, unfortunately.
A similar problem applies to people who try to change the default Windows font. The dimensions of windows and dialog boxes were created for the default font. When you change the font, the results are unpredictable.