On Thu, May 7, 2015 at 12:12 AM, Matt Corallo <bitcoin-list@bluematt.me> wrote:

The point of the hard block size limit is exactly because giving miners
free rule to do anything they like with their blocks would allow them to
do any number of crazy attacks. The incentives for miners to pick block
sizes are no where near compatible with what allows the network to
continue to run in a decentralized manner.

Miners can always reduce the block size (if they coordinate). Increasing the maximum block size doesn't necessarily cause an increase. A majority of miners can soft-fork to set the limit lower than the hard limit.

Setting the hard-fork limit higher means that a soft fork can be used to adjust the limit in the future.

The reference client would accept blocks above the soft limit for wallet purposes, but not build on them. Blocks above the hard limit would be rejected completely.