Is there a reason why miners should respect the default maximum block size? Actually the default maximum block size is 300,000 Bytes and the protocol hard limit is 1MB.

So what I understood is that all blocks over 1MB will be rejected but the ones between 300,000 Bytes and 1MB will not.

So if there are lot of transactions with little fees that make that a miner can create let's say a 700,000 Bytes bloc. Will it be profitable for the miner to create such a block or is there a additional risk that his block will be rejected?
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.