I think browsers are somewhat unique even among "large projects" for the sheer breadth of their codebases. Everything from networking to graphics to midi is somewhere in there, and they all have to provide a somewhat common interface. It just has to be complex, no matter how well the project is organized.

On a sidenote, firefox has fantastic build times (like 20-45 minutes cold), and the codebase is fairly well organized, so if you want to try contributing to a browser, that's probably the one to start with. Chromium has a much nicer source IMO, but cold build times can be as high as seven or eight hours.