Mozilla is enormously bloated - but skimming off useless "features" and leaving the browser itself should slim it down a bit.
As for other Linux browsers, porting them all suffers the same problem - they have their own GUIs, and don't use the underlying OS, probably because Linux is so cryptic and unfriendly it doesn't have a standard GUI. So you have to port the GUI too - and thats another reason why the ports are big, bloated and slow.
