I use the same OS with latest Mixxx 2.1-alpha-pre (build r5590) self-compiled.
Additionally, the same behavior with Mixxx 2.0.0 (build 1:12 r 5772) via deb package on another clean machine with Trusty.
In the mixxx preference pane on both machines are these settings:
row height: 20 px
font face/height: Ubuntu 11 pt
The default values, I think.
On the analyze pane the measured row height is constant 30 px.
On which OS are you?
I cannot confirm this on Ubuntu Trusty using Mixxx 2.0