
Trying to have him form the whole head would result in a comically tiny beast noggin

I personally think the beast mode looks good, probably as good as could be got on a retail toy without throwing out either the jet mode or the resemblance to the classic design.
And they did a good job consolidating the robot legs and beast mode tail/jet fuselage; compare to the original toy where the robot legs are just hanging off the T. rex chest:
