The documentation clearly claims that "On WiredTiger, compact will rewrite the collection and indexes to minimize disk space by releasing unused disk space to the system." However as far as I can tell in 3.0.7 this does not actually happen. I reduced dataSize of a collection from 130 GB to 30 GB and the storageSize remained the same at 61 GB after running compact. This was mentioned in SERVER-19062 which was marked as a dup for a ticket that doesn't address this problem.