OK so the new model is awesome, and huge, and I think I understand why they went in this direction. Two armies at the moment have griffons, the High Elves, and the Empire.
High Elves: They get their griffons from the Annullii mountains. I believe that the griffons, while tame, are allowed to roam free, as do the high elf horses. There are no stables and breaking in of the beast, they are free range. When the lord needs to ride his horse or griffon, he calls it, and talks to it in animal speech, and off they go. Its more of a contract of use than a master/beast relationship. Thus the griffons would be closer to their natural form, so example a mustang horse. Smaller and faster.
Now to the Empire. They have been using griffons for a long time, but do not have the animal speech and rearing skills of the high elves. The griffons are kept in stables, and probably broken in, much like any horse would be. Since they can't just sing to the air and have their friends arrive, they are are bred in the stables. Like any breeding program that lasts at least 2500 years (Sigmar's time), it has a severe impact on the animals themselves. As armour got heavier, the animals needed to be stronger and bulkier, and selective breeding would have brought this about. Where High Elves ride Mustangs, the Empire ride those massive muscled Norwegian horses x 10.
It makes sense to me, and although the stats are the same, a human warrior and an orc have the same strength stat, so its not such a big deal