They cannot handle 11.6MB unconfirmed transactions then how will they handle 20 MB blocks with 200 MB uncorfirmed transactions ?
It would be dramatically more expensive to spam the network with 200 MB of transactions.
plus, miners have the power to choose to process the spam or not. process it, make lotsa fee income thus strengthening themselves (which is the last thing a spammer wants), but also risk getting orphaned with given today's connectivity rates. or choose not to, protecting themselves from an orphan, and continue processing small efficient blocks for maximum propagation.