Is there a reason why miners should respect the default maximum block size?
You should know that as of bitcoin 0.8.6, the default maximum block size is actually now increased to 350,000 bytes.

The 350,000 bytes is a soft limit. All miners are motivated to maximize filling the block size to 1MB for each block because it would be irrational for them to do any less and leave fees to other miners that they could easily collect themselves.

Thus all blocks are usually mined to the maximum limit with paid transactions and then free transactions are included once all fee-based, unverified transactions have been verified.