Fender Japan was producing the Contemporary Stratocaster in the years leading up to the HM (with locking trems), so I've always assumed it was a natural progression. Here's a picture of a Contemporary if you'd like to make a comparison.
i wonder if the rationale was something as simple as making it look more reminiscent of a jackson or something? if the HM model was a Fender "reaction" to losing market share to super strats it could be something that silly. i love the shape and feel of this guitar though.
Yes the body of the HM is the shortest, and also the narrowest than both others. Smaller indeed !
Difficult to put a rationale behind a looking design, but my clue brings to the Lead shape, and as a reminding both also shared the same pickup configuration : one single humbucker for one HM model and the Lead I.
This let quite an important part of the body empty, that can be weird regarding the large 62' shape.
Thus a shorter body helps balancing the "filled" and "empty" places. The designer has certainly taken the Lead as a model looking for how to solve the single HB issue visualy speaking and he adapted a Strat looking over the Lead dimensions.
That's my purely design constrainst interpretation ;D