Mostly an indexing issue, affecting the ranking for minor keywords. On Google several million pages from the sites are indexed, so the sites are (correctly) found for tens of thousands of obscure terms, tens of thousands of entry pages.
MSN also crawls many thousands of pages a day, but the number of pages indexed (after a year) is a few thousand (on one site) to a few hundred (on another site). Which is why the MSN traffic proportion is higher for small sites where they are more likely to have indexed most of the pages.
It may also be related to volume on MSN, as one site is #1 for the main keywords on MSN and #20 on Google, yet MSN sends almost no traffic.