apache2-mpm-worker memory usage

I run a _very_ low-performance Web site off a 128MB ram OpenVZ VPS and
Apache2 with the prefork mpm. When I've tried installing the worker
MPM (with very low settings), my memory usage immediately jumps up to
the 256MB burstable RAM before Apache quickly dies. How can 2 multi-
threaded Apache processes be using up that much RAM, when 14 separate
processes use up around 75MB? I think I'm probably missing something
important here.