Although the model may be slightly smaller, this clip doesn't make the mumak look like it is double the size or big enough to hold a tribe on it. IF Legolas = 1" model, the mumak is big, but I would not make his foot as large as a cavalry model, it just isn't so, even when they show the mumak step on a rider is it only the rider and the front of the the horse that is crushed (but the whole thing goes down so it can appear like the foot is as large as the whole thing). Also the low angle shots make them look more towering as was part of how PJ shot that battle, to show how it looked from the terrified riders trying to fight it.
But as for the Smaug model, I think people are wathcing the trailer with "big eyes" in wonder and excitement. Smaug may be large, but I would not go so far as to say he is as large as people are making him out to be in quick shots with a hobbit using perspective angles as is done in alot of LOTR/Hobbit (which makes it tough to copare sizes and easy to shoot people to be taller or smaller looking).
it is very tricky to get a acurate size guage of the mumak, I think the model needs longer legs by a tad bit.
This one looks good,