Personally I think the thread name should just be "uncommon sizes".
1280x720
1280x1024
1440x900
1600x1200
These are all common sizes too.
Older sizes like 800x600 or 1024x768 and unusual these days but you still get them. You'll also find a lot of people with unusual sizes. Like my Windows install uses 1824x1028. Things like that. No matter what they are, I wouln't call them "undersized".