That's why webapps are built in a modular way: it's similar to 3D games in the sense that you don't need to have everything immediately to start using it: load what you need to display first, and get the rest when needed: e.g. you don't need the last level of a game when you're just starting it.
Actually, I was toying lately with the idea of making my own modules loader * insert mandatory xkcd strip about competing standards * because I wanted an hybrid loader that can pack multiple modules per file, but that doesn't have to have all modules in a single file either (e.g. 3d models could be packed with their textures, or levels packed with the models they need) as I haven't come up with a satisfactory setup to do so in r.js or browserify yet, however Unity already solved that issue in its Web Player, so making a JS version of it should be trivial.
Update: I'm actually having high hopes for webpack as it seems to combine the strengths of both Browserify and RequireJS, and seems tweakable to package modules into a couple of files. I'm not sure why it's way less popular on Github though.
Actually, I was toying lately with the idea of making my own modules loader * insert mandatory xkcd strip about competing standards * because I wanted an hybrid loader that can pack multiple modules per file, but that doesn't have to have all modules in a single file either (e.g. 3d models could be packed with their textures, or levels packed with the models they need) as I haven't come up with a satisfactory setup to do so in r.js or browserify yet, however Unity already solved that issue in its Web Player, so making a JS version of it should be trivial.