When using proportional font sizes, the size on preview is governed by the default font size set in the browser preferences - which is usually 16px in most browsers.
Freeway will also display this font size at 16px in the design view so if the default has been changed in either the browser or Freeway the sizes will vary.
It's generally a good idea to use fixed pixel sizes where possible so you have more control the text size in the browser.
A few things to note are that increasing the text zoom/size in the browser menu will also
increase/decrease the size of text on your page, there is nothing that
can be done to prevent this happening.
Also, text size on Windows is always slightly larger than on Macs, this is because Mac OS keeps the font as true to the original as possible so what you see on the screen is what you get in print. Windows changes the font by forcing it into a pixel grid to make it what they consider to be "more readable" on screen. The best way to handle this change is to work with it by expecting it to happen. A good way to do this is to make your items table based (the only available option in Express) by unchecking the Layer option in the Item Inspector, or using a relative positioned CSS layout, as outlined in the CSS Layout in Freeway article.