http://research.microsoft.com/pubs/172840/Bitcoin-Red-Balloons-SIGEcom.pdf
Read that paper to learn more
And here's a reflection on it by MIT Media Lab
http://research.microsoft.com/pubs/172840/Bitcoin-Red-Balloons-SIGEcom.pdf
Node incentives are important, and likely play a role in dismissing certain concerns regarding issues inherent in larger block sizes by potentially representing a more Symbiotic relationship between miners and the nodes that propagate the transactions to them.