Shared libraries

Why do some apps like Chrome and Firefox have tiny main executables and pack everything in a big shared library that contains everything? It's not like there are other users of the library, and at least on some systems executables should perform a bit better (no risk of needing to be relocated on load).

Other big programs do the logical thing and provide a large executable and no shared libraries. I have even seen some that changed the policy from version to version. What I'm asking is why would you put anything that is only used by a single program outside the executable.

Could be a way to separate some platform specific launch code from the main program logic, or to load other stuff in parallel while you're waiting for the main program. If Chrome does it, there's probably not a noticable performance penalty, because I'm pretty sure they've tweaked the hell out of the startup.