Sorry - should have mentioned that..
I'm using ESET Smart Security - Ver 9.0.408.0 /DB V14618
Windows 7 x64 @ 24GB memory.
Interesting read. I've found that it usually runs around around ~140mb then slowly creeps up to around 240mb~ after some time. I don't think I've seen it go past 250mb yet. It's not a massive deal as I've got 24GB RAM, it's just that I've noticed the change of performance.