On one hand, mstone is complaining because Mail downloads every message. And on the other hand he is complaining because he thinks iOS Mail does not. Of course, with IMAP they both stay in sync with the server, as they should.
I suspect you don't understand what you see in Activity Monitor. My Mac has been on for a few weeks, and I run Safari all the time with lots of tabs. Currently I have four tabs open and Safari is using 292MB of RAM. I just launched Notes and it's using 35MB.