The negative number after the font name is the size. It's a negative number because of the way fonts work in Windows. It isn't supposed to be the font's point size but is in some other units.
If I copy your fonts section over the one in my settings export, and then import those settings, then I also get Tahoma 10pt fonts.
However, if I go into Preferences and change all the fonts to Tahoma 8pt and export my configuration then I get this:
<fonts>
<files>Tahoma/-11,400,0,0,0,1,0</files>
<tree>Tahoma/-11,400,0,0,0,1,0</tree>
<status>Tahoma/-11,400,0,0,0,1,0</status>
<header>Tahoma/-11,400,0,0,0,1,0</header>
<thumbnails>Tahoma/-11,400,0,0,0,1,0</thumbnails>
<foldertabs>Tahoma/-11,400,0,0,0,1,0</foldertabs>
<fdb>Tahoma/-11,400,0,0,0,1,0</fdb>
</fonts>
Importing that again still gives me 8pt fonts so Opus seems to be working correctly on my system.
Could you set your fonts to 8pt again and then export your config and double-check that you're still getting -13 and -14 instead of -11 for the size?
Do you have any options turned on in Windows or the display drivers which affect font size or DPI? (e.g. A "Large Fonts" option.) Are you exporting the settings on one machine and importing them on another, where the font/DPI settings are not he same on the two machines?