Chromium is slim? Compared to what exactly? The only browser I've ever used with similar memory usage to Chromium would have to be Opera. Even on a relatively optimized build of Chromium 7.0.510.0, it's pretty typical for me to see memory usage of ~300MB with only three tabs open. I shudder to think how much memory Chromium eats when used by tab junkies who have 20+ pages open at a time.

I can see how Chromium gives users the illusion that it's slim, but I don't think it's accurate to call it that. I would reserve the word slim for browsers like uzbl and Midori.