Check the font being used. Especially in the original. Linux lacks most of the commercial fonts folk like to actively specify, so family substitutes are used (as specified if they are specified). Often very poor choices result. For example the font Arial is a Sans Serif font, so is Courier so at time you can see one replaced by the other when Arial is not available.

At a more technical level, implied font families are used in substitutions where a list is not provided. This site provides some background. https://www.w3schools.com/cssref/pr_font_font-family.php

Thx for the reply!

It looks like this is the font they are using....''font-family: Helvetica Neue, Helvetica, Arial, sans-serif; font-size: 16px;"

I installed all the Helvetica fonts I could find and it did change the font but not the size of the font.

Is it possible to have TB display all fonts in message replies the same size?

See how they specify a size. If they were children you would slap them on the wrist and say "don't do that little johnny". You entire discussion here is dictated by your correspondents choice to force you to see the fonts in a size of their choosing. Not a correct size for your system which will be automatically derived unless someone forces a size (in this case 16 pixels, approximately 12points and comfortable to read when printed on a piece of A4 paper)

I am not sure if the "allow messages to use other fonts" in the Fonts and Colors advanced button will force the size as well. But I guess it would be worth a try.