php-cgi processes, while being used by the Apache server, are not part of it. I suspect you are asking about reducing Apache's memory usage when it is not what is actually consuming your memory... try to investigate what else is using up memory (Apache? or PHP? or whatever?).
To answer your question about reducing memory usage of the MPM, there isn't much you can do to reduce it while keeping up with the same workload - the MPM will use whatever it needs to do its job.
You can shrink it a little by deactivating unneeded Apache modules, but that won't make such a big difference.
Maybe you have too much idle threads (SpareThreads in apache's lingo) but again, you either actually need them to properly handle spikes in incoming requests or they'll do little difference on memory consumption - unless you did something really really strange with Apache's config.
You didn't give enough information about your current configuration, memory consumption, workload to speculate more on the subject. Maybe also a look at the Apache documentation would help, did you read it before asking?