Express has a limited number of remote peers, which limits the total datastore size. The datastore is 13GB per peer. For each peer FIFO happens when the datastore is full. In other words when the peer datastore is full, new traffic will swap out old traffic data.
Size of the database is actually not crucial. A 4GB datastore can provide better compression than a 40GB store. If the 40GB has 400 x 100 Mb files that can be matched, optimization is going to be close to 0% because it is unlikely that these files will be transferred multiple times. Now if the 4GB had 32 Million 128 byte patterns, then there are constantly hits.