HTC's market capitalization is much larger than Nokia's.
That's true.
Android is their bread and butter and Android is hot right now, whereas Nokia has been on the decline for a while and Windows Phone is really only just beginning.
Windows Phone is probably not going to see the explosive growth Android did because Microsoft has stricter rules and isn't going to whore the OS out, in terms of letting any carrier and any OEM do whatever they want with it on any hardware regardless of the end user experience. Microsoft also doesn't have the benefit of the wide open field that Apple left for Google to exploit.
Nokia is taking a calculated risk here, that Windows Phone 7/8 will experience steady growth and that it will enable them to differentiate their products from their competitors, rather than taking the easy way and going with Android like everyone else.
It could go really well, or it might not work out, but I respect Nokia for having the integrity to go "all in" on this deal and that's one of the reasons they got my business.
I'm taking a risk by signing a 2 yr contract on a phone OS that barely has 3% of the market, but Nokia is right there taking that risk with me, they have some skin in the game. I don't believe for one second that HTC or Samsung would have had a software fix for the data issue Nokia just did in a WEEK and refunded everyone $99, for example. They'd have no reason to bend over backwards to make 0.5% their customers happy, they are too busy catering to the Android heathens.
I will admit, I have a strong bias (maybe even borderline hatred) of Android, so the fact that Nokia told Android to take a flying f*ck earned them my eternal gratitude. Someone had to have the courage to do something DIFFERENT.