This a very annoying bug that has been around for years. Please log a bug report with Adobe: https://bugbase.adobe.com/
As a workaround, I use 2 CSS files. One with points that I use for working in RoboHelp. And one that I manually add to my output to overwrite the fixed size CSS. No easy fix I'm afraid.
We all just ignore what it appears like in Design View because we know that’s NOT what it’s going to look like in our output.
Thanks, Willam. Reported as bug.
Using two CSS files is probably the best thing to do - I already have two files to keep RH from messing around with <pre> tags and stuff.
@Jeff: True, but it's hard to ignore huge ~80px headings when writing topics. I could do it all in HTML view, but not everyone in my department can.