After some extensive calculations and use of several spreadsheets, we here at Massively determined that $5 billion is not, in fact, "five times" $3.8 billion. So on one hand, it's all about who is crunching the numbers and how they're crunching them, but on the other hand, there is no doubt that the Chinese gaming market is huge.
At first glance, the fact that a large percentage of the Chinese population is on a limited income makes it a bit surprising that the MMO market is booming. However, it's this very thing that is helping make it so successful: business models based entirely on microtransactions are the norm there, with individual item purchases hovering in the $1-2 range. Such a small amount makes less of a dent in a thin wallet.
The full story is an interesting look at the Chinese gaming community, and well worth a read.