I notice no-one has touched on how em works as a font size unit, so I'll see if I can clear that up for you:
There are two ways to tell the browser how big to make your font. One way is by setting an absolute size, and the other is by setting a relative size — em is a relative size.
For absolute sizes, like px or pt, you're telling the browser exactly what size you want the text to be. If you write 12px, it's going to come out as exactly 12 pixels tall. For relative sizes, like em, you're telling the browser how big to make the text with respect to other text on the page. This works sort of like a percentage, so if your footer text is sized at 0.8em, it will be rendered 80% as tall as the main text on the page.
This explains why you're getting different sizes on different pages. If the font size of the body is set explicitly on one page, but not set at all on another (or set explicitly to a different size) that will make your em-sized fonts render at different sizes.
This is also why it's really bad to use <font> tags. It's going to be a serious pain to dig around those tags and figure out what explicit sizes are being set that might be throwing off your ems. When you set your font sizes using CSS, this is not only easy information to find, but also easy information to change.