Why aren't miners filling up Bitcoin Cash blocks? I see that there are over 100K transactions in the mempool, but most blocks have only a few hundred of transactions.
Can anyone shed some light on this?

The blocks can be seen here: http://blockdozer.com/insight/blocks

Clarifications: 


I'm running Bitcoin ABC. 
getmempoolinfo returns "size": 101223
getinfo returns "version": 140400
At the time of the question, Bitcoin Cash mempool had 917 transactions, and the Bitcoin mempool only had 8.1k.


  yes from ABC getinfo says "version": 140400


You're running an outdated version of Bitcoin ABC. 140400 translates to Bitcoin ABC 0.14.4, but the current release is Bitcoin ABC 0.14.6. Mandatory two-way replay protection was introduced either with 0.14.5 or 0.14.6. Your older version still considers Bitcoin transactions to be valid, so you're collecting both Bitcoin and Bitcoin Cash transactions in your mempool. Obviously, Bitcoin transactions aren't leaving your mempool though, because they are not being included in blocks. Thus, eventually your node's mempool will consist solely of Bitcoin transactions, assuming that Bitcoin transactions generally have a higher fee level.

Updating to the current version of Bitcoin ABC should resolve the discrepancy.