maybe, it depends if there are holes as well, antec 900/1200 have lots of holes and are good at keeping stuff cool
You should have more room inside the case for air flow around the components but it depends on the case having good air intake and exhaust capability. Are you thinking of a specific case?
Doesn't always work that way, but it is generally the case - no pun. More important is good air circulation, which is done by vents and fans. If maufactures get the vents and fans at the right size/place/speed, it shouldn't matter what size the case is.
Main reason for it generally being so, is you have more room for fans etc, but yes, and the 1200 is riddled with holes so you have less restriction!
Rule of thumb: cold air in at bottom front, hot air out top back. Have a look here and here for more guidance/discussion on the matter.
