Webmin uses ~11MB on a 32 bit system. It used to require an additional ~11MB for the data gathering cron jobs, backup cron jobs, etc. but now it runs most of those tasks within the Webmin application service process and so only increases usage slightly during those tasks.
If that's too much, you don't need to use it. (You could shut it down when you're not using it, though it loses a lot of its utility when it can't communicate with your slave DNS servers, keep resource usage data, etc.)
Virtualmin is a more complicated beast, and can take quite a bit more memory, if caching is enabled, but that is optional, and when disabled it takes the same ~11MB as Webmin.