LRUFilesCache grows arbitrarly

We are experiencing memory issues in our system where we have to scan very big directories. After doing some investigation it turned out that the cause of the issue is the caching of vfs. (http://mail-archives.apache.org/mod_mbox/commons-user/201312.mbox/browser)

From this reason we tried out LRUFilesCache, but it seems this cache can still grow way beyond the limit.
This can be reproduced with a simple test code:
https://sourceforge.net/p/screeningbee/code/HEAD/tree/trunk/vfs-cache-test/src/main/java/ch/systemsx/bee/vfscachetest/CacheSizeChecker.java

In this example we set the max size of lru to 100, but running a while this example and then checking the heap of the program showed that the real size of the map can be 10 000 or more too. And it seems its growing constantly.