MacBook :: Web Browser Leakage Uses Up Almost All Memory?

I consistently find that when using Safari (or Firefox), within a short period of time (sometimes even less than one hour), the browser has taken over nearly all available memory. This happens even when the browser is the only program in use, and when no webpages with extensive audio or video content are open. 

Can someone explain to me why this happens -- i.e., why web browsers seem to hog all the memory they can find -- and (more importantly) whether I can take steps to prevent it from happening?

As an example, my current System Memory readings (from Activity Monitor), with only 'Firefox' and 'Activity Monitor' open, are: Free memory 74.9 MB (but frequently dropping to less than 10 MB) Wired 228.4 MB Active 1.20 GB Inactive. 504.5 MB Used 1.92 GB VM size 31.99 GB Page ins. 292.1 MB Page outs 120.6 MB Swap used 518.7 MB

Info:
MacBook, Mac OS X (10.6.3), 1.83 MHz