Hey,
I think Niklas his comment is spot on:
Rather than absolute size, we should care about core that is modular
and has well-defined interfaces. Moving stuff to extensions is one way to encourage this, as then the functionality cannot be relied on or it can be replaced with an another extension.
MediaWiki is currently closer to being a ball of mud then a set of core components nicely separated and working together via clean interfaces. We have so many components that could just as well be independent from the rest of the codebase, but they are not. Loose from the direct technical cost of this, it also prevents reuse outside of MW and prevents using other third party libraries from being used for those components rather then custom MW code. Though we can clearly not just get rid of this legacy (short from rewriting MW), we can encourage independent code and clean interfaces.
Cheers
-- Jeroen De Dauw http://www.bn2vs.com Don't panic. Don't be evil. --