If you get the 4900mah FlightPower packs, you'll notice that they're much smaller and narrower than the 3700mah.
Also, for those who want to buy the Evo20s from the US, this is a site you can visit:
www.flightpowerusa.com
It is actually called atlantahobbies, but once you click on the link I gave you, it will automatically direct you there.