[Zope] understanding Zope's memory usage

I am running a server with a handfull of plone intranet sites. The server has 4GB memory, but is now starting to page. What I notice is that the different Plone instances vary widely in there memory usage. For one site every process uses 400Mb, and there are 6 processes running! That is 2.4 Gbyte for one site! I see there is one deamon process that is small, and 6 processes that are big. If we call the six biggies a-f: deamon start a a starts b b starts c d e f c d e f are probably zserver threads, but what do process a and b? And why do the all have the same size and RSS? What are the alternatives to limit the memory consumption? Anybody who can enlighten me on this? Thanks, Lucas