Default Character Width Size
Different platforms use a different default character width size.
You'll notice that the same text and graphics will look slightly larger on a Mac than it does on a PC. This is not your imagination!
The two operating systems use a different default character width size. On the Mac, each character pixel is sized to be 1/72nd of an inch at the default 640x480 screen size. On the PC, each character pixel is sized to be 1/96th of an inch at the default 640x480 screen size. This means that a 72 pixel wide image will fill up one inch of Macintosh screen real estatebut not quite an inch of PC screen real estate.
Make Liquid Pages
People are using different screen sizes and viewing areasliquid design helps your page look good on all sizes and viewing areas.
When you create a graphic treatment, you tend to view it on your monitor, under your viewing conditions. Most readers won't share these same conditions. That's why it is important, when possible, to design liquid pages.
A liquid page is one in which the page elements will flow into whatever size screen your audience is using. Just as you wouldn't want to be forced to keep your browser at someone else's idea of an ideal size, neither should you force your readers to keep their browser at one size!
Be aware that images will display at slightly different sizes on a Mac and PC.
To keep your page liquid, avoid fixed width tables and oversized imagemaps.