What is a sizeable chunk? And how is this information measured?
I also dont think the Xbox Live integration on WP is that big of a draw. Every gamer I know (those who play hardcore and those who play casually) owns an Xbox of some sorts. Earning achievements is one of the few things I tout about WP. And yet, they absolutely show no interest other than slight amusement. I've also seen that a number of gamers are so elitist, not even achievements will get them to acknowledge mobile gaming as a valid form of gaming.
A sizeable chunk is one that is large enough to effect best-seller lists in the app store. The number of Xbox gamers on Windows Phone is large enough, in terms of who actually buys games from the Windows Phone Store, to affect store rankings.
It's not as big of a draw as it was at the start of WP7 when Microsoft seemingly was all-in on supporting mobile gaming on Windows Phone, but it definitely still is a big deal for a lot of gamers. There are many active online communities where you can track this information.