Size of an antivirus application depends on the underlying engine, & the code, along with all the features
Norton compared to Kaspersky or NOD32 is big, but u guys are also forgetting that norton Antivirus 2006 has far many more features compared to the other Antivirus, such as worm blocking, network protection etc
some of these features are also in the other AVs, one thing that I would say indeed, that sticking to Windows Native UI & interface helps in reducing the overall size
the code of norton antivirus is age old, which is the biggest reason for it to be a bloat, now with Vista changing it's networking stack, I hope, Norton gets a whole re-write of it's code, with optimization in mind
I use NAV 2005, got 1 GB RAM, & it just sits inside the system, I don't care how much RAM it eats, I disable it when not online anyway, to free up CPU cycles for other things