[They are the two most used resolutions on Macs]

They are the two most used resolutions on Macs and third and fifth most used on windows, respectively, acording to this page:
http://unity3d.com/webplayer/hwstats/pages/web-2011Q3-desktop.html
well, at last from the people who used unity web player... =P
But this is a pretty reliable source. 

I tried -fullscreen -1280x720, but the screen got misaligned.

dont know if this is supported on windows, I am using a mac.

Thankyou.