There are settings within IE to override the font sizes on web pages. I don't think you would have set them accidentally though.
To change webpage text size
Open Internet Explorer.
Click the Page button, click Text Size, and then click the size you want.
To override website font and color settings
If you want to have the fonts and colors you specify in Internet Explorer to be used for all websites, regardless of the fonts that have been set by the website designer, follow these steps.
In Internet Explorer, click the Tools button, and then click Internet Options.
On the General tab, click Accessibility.
Select the Ignore colors specified on webpages, Ignore font styles specified on webpages, and Ignore font sizes specified on webpages check boxes, and then click OK twice.
Hope that helps