Normally, memcached uses only as much memory as it needs, even if you give a maximum limit. It's not like the Linux system buffers that by design can expand to fill all the available memory in order to improve system performance.
You can telnet to your memcache and issue the commands
which should give you an idea about how many objects of which sizes are in your memcache and how much memory they occupy.
If memcached does not have enough memory to store all your objects, it will evict older objects, which will increment the "evictions" counter returned by
slabs. If "evictions" is at 0, then your memcache has enough memory.
You can find many other useful commands and solutions at "the memcached cheat sheet" at http://lzone.de/cheat-sheet/memcached