Does the mempool size equal roughly the unconfirmed transactions? My bitcoind (v0.13.0) mempool size is currently 12k transactions as can be seen from: 

tail -f ~/.bitcoin/debug.log

I find this number very surprising as I naively expected it to keep in line with the number of 'Unmatched transaction' as reported say by blockchain.info which currently shows 3k transactions.

https://blockchain.info/unconfirmed-transactions


What am I missing?
Every node owner can set their own policy for the mempool. The mempool is limited two-fold: 


With -maxmempool=&lt;n&gt; you can set an explicit limit of MB that it will not exceed. The default is 300MB.
Transactions that don't surpass the minRelayTxFee are not added to your mempool.


As statoshi.info has the mempool at 145MiB and ~11k transactions, it appears likely that different settings on the minRelayTxFee seem to cause your and blockchain.info's divergent mempool size here.