By far, the most obvious difference between Mac and Windows Web page is the size of fonts. Any font size specified on a Mac will turn out at least 33% bigger on a Windows machine, but the GIFs and JPEGs stay the same size. When the font size increases by 33% the lines wrap differently and blocks of text take up more room on the page.
Unlike print design, where 10 point type is always 10 point type, a Web surfer has the option to set whatever default size and font they like, so Mac/PC differences apart, it is not a good idea to assume that the type size that you see on your machine is what your reader will be seeing. They might have a 20 inch monitor with their browser maximized to fill the screen. They will also probably have Windows set-up to use 'large fonts' which are even bigger again! But still, the graphics stay their original pixel size.
There go your proportions!
The converse situation is where a PC-based Web designer chooses to use a very small font, which looks fine on the PC screen, but is too small to resolve properly on a Mac screen. The minimum size for a Mac font is nine pixels high, if you try to use eight or seven point type, there are not enough pixels to resolve the characters. Nine point type on a PC is equivalent in size to twelve point type on a Mac and seven point PC type is really nine pixels high and resolves quite satisfactorily.
The principle of using relative type sizes like -1 and -2 should prevent a browser from trying to display type that is too small but just try looking at Microsoft's Knowledge Base on a Mac using Netscape and you will see what I mean.