To be fair to Apple I think this particular issue might be buried in the TinyMCE editor, and it has only become so apparent because of the new choice of font size. I think we were on 14pt before and difference to 13.3333pt was much harder to notice.
Here goes a live CSS experiment. I've just tweaked this page to show the editor text at 13.3333pt and I'm going to shuffle a few words around as before to see what happens: Cart Horse . How did we do?
Argh, the wrong direction, let's have another go with the editor set to 8pt: Cart Horse. Can you guess?
Still wants to resize (10.6667px this time). One last thought is not to set the font size for the editor text at all. Third time lucky: Cart Horse. Bingo!
So this line of CSS stops the site telling the editor what size text to use, and in turn anything copied and pasted within the editor shouldn't acquire a size that hadn't already been set.
.tiny_mce_content {font-size: inherit !important} /* Prevent cut & paste scale errors in TinyMCE */
Another line to go into my Minimal CSS tweaks for ASC.
tt2