Your problem is that you have a fixed (px) line height, when you should actually be using a percentage (%) line height. Line heights of 150% or greater are recommended, since they help people with dyslexia read properly.
Simply change that one line to:
line-height: 150%;
and you should be fine when changing the font size.
For more, see “Success Criterion 1.4.12 Text Spacing (Level AA)” from the Web Content Accessibility Guidelines (WCAG).
Anyways, that should be a better and simpler fix.
Enjoy! 