It is very likely because you don't have the fonts that Windows incluldes on your Linux box.
For example, Windows includes things like "Arial" and "Comic Sans". Linux doesn't have these (at least by the same name).
Web pages specify fonts from most specific to more general alternatives. For example, on news.google.com, they specify:
font-family:arial,sans-serif
This indicates that the browser should first try to use font "Arial". Windows includes that, so that is what will be used on a Windows box. On a Linux box, it looks and likely sees you don't have it (unless you installed it yourself, for the most part). In that case, it looks at the next option, which is "sans-serif", so your Linux browser will use the generic Sans Serif font that it has.
That explains the difference.
It is possible to have Linux use your Windows fonts. If you dual-boot, I believe you can even have a symbolic link to your Windows folder with the fonts, and Linux (technically, X) will be able to include them as available fonts.