I am also experiencing this issue. It is a bug and nothing to do with preferences.
By posting a message to a group of which I am a member, I have been able to compare the content of a message as sent with the same message as received. One paragraph in the received message was a huge font size compared with the same paragraph when sent. The HTML mark-up preceding the original paragraph is as follows:
<p class=3DMsoNormal><span style=3D'font-size:11.0pt;font-family:"Arial",sans-serif;mso-
fareast-language:EN-US'>
The HTML mark-up preceding the paragraph as received is as follows:
<p class=3DMsoNormal><span style=3D'font-size:110pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'>
You will note the difference. "11.0pt" compared with "110pt". Somewhere in the processing the "." has gone missing.
So, Microsoft, please get on with sorting this one out.