There seems to be at least 2 miners/pools not mining full one megabyte blocks while mempool has lots of transactions.

Telco 214 is always mining 749k blocks.

https://blockchain.info/blocks/Telco{88c91daedd271a990a10650c05d769cae2765e0603edf30ca95f18704e5748e8}20214

Antpool is not always mining 1 megabyte blocks. Looking at blocks from 450959 to 451634 Antpool mined about 80 full blocks but seven 99k blocks and thirteen blocks with either 269k or 369k bytes. Strange.

https://blockchain.info/blocks/AntPool