Neither the Nexus tablets, nor the Surface ones had anywhere near as many sales as Apple's iPad (in the tens of millions), but at least the Nexus 7 sold a pretty big number in 2012, somewhere close to 6 million units. That also happened to be 60% of Nvidia's Tegra 3 sales last year, which is why it's such a big problem for them that Google is going with Qualcomm for the next-gen Nexus 7.
The Nexus 10 tablet didn't do as well, and is rumored to have sold only around 680,000 units. That would put it about on par or slightly less than Surface RT, and probably around the same number of sales as Surface Pro, or a little more than that. It's hard to know for sure, since Microsoft lumped Surface RT and Surface Pro together, when they mentioned the sales numbers earlier this year.
What makes a lot more sense in this case, though, is to compare all the Nexus tablets (including the Nexus 7) with all the Surface tablets. Or just compare them side by side, with each individual tablet. Then we get a more clear picture of how each has done in the market.
One thing is for sure, though, that neither one of the Surface tablets, nor the Nexus 10 did all that well in the market. I feel that Google could've done a lot more with the Nexus 10 if they had used a better design and build material, and also do a much better job at promoting it.
Android still doesn't feel very optimized for 10" tablets right now, so that probably played a part in it, too. Hopefully with the upcoming version of Android, Google will be making at least the OS interface look a lot more optimized for 10" tablets, even if it will still take a while longer until we get a ton of apps that are also optimized for tablets.
They've recently started giving Android developers more incentives to create tablet apps, which should've happened a long time ago, such as being able to post the screenshots of their tablet apps on the Play Store. But that's hopefully only the first of the many incentives Google is going to give developers to improve their apps for tablets. The iPad has a very significant lead in tablet apps right now, and Google is not going to catch up with Apple in tablet apps if they are sitting around and doing nothing about it.