If you are know how to use your browser's JavaScript console, you can now get an ordered list of all ResourceLoader modules that are loaded on the page, sorted by the total size of each module's JavaScript and CSS assets -- simply run "mw.loader.inspect();".
It works best in newer versions of Chromium / Chrome, where it takes advantage of the availability of console.table(). It looks like this: http://i.imgur.com/zGymrsF.png
In the course of testing this feature yesterday, Matt Flaschen spotted and fixed redundant module loading in TimedMediaHandler (see bug 55550). That's pretty cool, no?
Do remember that size != performance, though -- just because a module is small does not mean that it is performant. (The reverse is also true.) This tool also does not account for factors like gzip compression. So no burning developers at the stake, please :) But curious poking is definitely encouraged.
Thanks to Timo & Roan for helping this along.
--- Ori Livneh ori@wikimedia.org