It might be because the font size is defined in the CSS in pixel units, instead of em, which is a point size (DPI dependent).

http://www.w3.org/TR/REC-CSS2/syndata.html#length-units

If you have a problem reading text on the screen, I suggest you use something like Clearly extension that reformats pages with a much more reading-friendly style.

You don't have to sign up to use this extension : https://evernote.com/clearly/

You can also try to print the page into a PDF and read it like that.