This is quite a nice innovation, but how do you plan on keeping the blockchain size small?
The message takes almost no space if not included.
At it's largest, it is relatively small, maybe the size of a couple of inputs in the transaction. So I don't see a big increase in block size.
Also, the message size adds to the transacton size, so a transaction with a larger message is likely to incur more transaction fees. That should deter misue to some extent.