The root cause of this issue seems to be with Outlook and it's poor HTML support. More specifically its poor support of CSS and inline styles.
As far as I can tell the GMail text editor applies font size using the style attribute on a <span> and font face using CSS by default which Outlook doesn't handle very well and applies a default size. The font face is applied using CSS which Outlook doesn't support very well (if at all) so Outlook uses what ever it has set as the default (may be Time New Roman).
Using the 'Default Text Styling' lab helps with the main text. As long as you don't change the font size in the text editor.
<span class="Apple-style-span" style="font-size: x-small;">asdasdasd</span>
Where as the Default Text Styling lab uses <font> which works better in Outlook
<font size="2"><font face="verdana,sans-serif">asdasdasd</font></font>
You can see this by clicking view source in the web browser.