Limits to cache growth / clearing?

I've set up a memcache server with 160 MB of cache available. It's been working rather nicely, and after a week there's over 80 MB of cache occupied.

My question is, what happens when my cache reaches the limit of 160 MB, when it gets full. I haven't seen the cache decreasing at cron runs or at clearing all caches with Devel, etc. It always increases.

Does it mean old entries are accumulating and cache clearing does not work? I'll appreciate somebody explaining how it works. Thanks.