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.

>>13
If the executable only has launch code, it can decide if it needs to load the library again, or just tell the existing process to open a tab.
Is there a load cost relative to the size of the executable which you have to pay even when other copy is already loaded? In any case this doesn't sound unreasonable.