If you compare Netscape Navigator to a modern browser, there is a massive difference in functionality. Just compare the HTML 3.2 Spec (51 pages when I do a print preview) with the current HTML Spec (PDF version is 1155 pages). That's a 20x increase in size.
Netscape Navigator did not have a DOM and did not have CSS! There were no dynamic changes of the document, no JavaScript modifying the DOM or style-sheets. No tabs. No audio or video. A modern browser is a vastly more complex program.