Aha. Yes.
We haven't switched the signature / greeting to the new editor, but we are able to use any already configured greeting / signature's.
The old editor used %% font size scales - relative to normal (x-small 50%, small 125% etc.)
The new editor uses point sizes (it's a bit more complicated than that, but...) - normal is 12pt, small is 10pt, etc.
So yes, the "old" and "new" size scales don't match.
I'll file this as a bug and see if we can fix this - maybe when using the "new" editor, we'll treat existing "font size" formatting "the new way", as "8pt - 10pt - 12pt" and not as "50% - 75% - 100%".
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In the longer term, hopefully in 1.15, we'll switch greeting /signature settings over to the new editor.
We'll also look at making it possible to "paste raw html" there.