Tiles are different sizes depending on their intended use. App tiles on Win8 are active - i.e. the app can render new information to them. A newsreader app can tell you most recent headlines at a glance (without having to launch the app), your email app can tell you number of unread messages, weather can display current conditions without forcing you to launch.
Larger tiles are used by apps that need to convey more information - your email app that just shows an unread count probably doesn't need more than the standard 1x1 tile, your newsreader might want a 2x1 to have room for headlines.
I'm not a huge fan of Metro, though I have been actively devving for Win8 for a few weeks. I don't think this is really a problem - all of this stuff is pretty obvious to users.
My experience has been that "Windows 8 Apps" default to the 2x1 tile size and all "pre-Windows 8 Apps" default to the 1x1 tile size that displays their "desktop icon".
So the rule could be: "All apps start at 2x1 if they have a 2x1 tile designed, otherwise they start at 1x1"
Larger tiles are used by apps that need to convey more information - your email app that just shows an unread count probably doesn't need more than the standard 1x1 tile, your newsreader might want a 2x1 to have room for headlines.
I'm not a huge fan of Metro, though I have been actively devving for Win8 for a few weeks. I don't think this is really a problem - all of this stuff is pretty obvious to users.