max_size on proxy_cache_path not working correctly (no replies)

I am having problem with max_size setting on proxy_cache_path. I trying to set a limit on disk caching space that can be used by nginx.
It works initially, but after a while the disk space used by nginx will grow much larger than max_size limit that I set.
Feels like nginx cache manager die or stop tracking the caching for some reason.

What could possibly caused this? Is there a limit on how many caching object you can have? I am running nginx version 1.7.6

Thanks