[Zope] How to hunt down memory hog?

I have a fairly simple Zope application which is occasionally (every couple of days) taking up around 75% of memory on my system. (according to the %MEM column of top) When this happens my application slows to a crawl, and I have to cycle my zope service. Is there an easy way to get an idea of all the active objects and their sizes, or else some other sensible way for me to figure out where this huge memory leak is coming from? Thanks, Mark Roach