Loads and loads of vessels are too big for current Panamax locks/canal (max size around 5.100 TEU). One of the vessel classes is post-panamax and there are various classes above that :-P. Panamax canal is now being expanded, but it'll only fit about 14.500 TEU vessels. The previous largest vessels Maersk has (E-class instead of triple-E class) also don't fit to the _new_ Panamax. That new Panamax is still underway.

See http://en.wikipedia.org/wiki/Container_ships#Size_categories