mongodb memory leak or big collection?
This has been asked and answered many times, so there is even a couple of FAQ sections on it.  First you will need to understand the difference between resident memory usage and the cache being full (very different things), then you need to look at how memory mapped files behave in general.  Here are a couple of starting points from the MongoDB Docs:

http://docs.mongodb.org/manual/faq/fundamentals/#does-mongodb-require-a-lot-of-ram
http://docs.mongodb.org/manual/faq/diagnostics/#memory-diagnostics

Short version, no it is not a memory leak, and yes it is expected behavior over time when the data set exceeds available physical memory.