HD vs. Polymorph

[Simulacrum]

It's easy to miss some minor details, so I thought asking would be a good idea.
Ok I know the Polymorph spell is very controversial in here but I'd like to ask a valid question.
Polymorphing allows you to morph into a form up to one size larger than yourself. (--> thats the easy part)
That means you can turn into an Ogre or a large Dragon (that means at max. a juvenile one)
I guess everyone will agree it's perfectly possible to do so.
But what if you turn into a Juvenile dragon but you have only 8 HD as caster? Do you have to choose a lesser form of that dragon or can you still be a large/juvenile dragon just with much less HD?
Of course everyone can morph into an Ogre with 8HD but can you assume the form of a large creature (the spell allows that) and just have the lesser HD?
Another example would be a Firegiant with 8HD?
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My guess is yes. Because polymorph just gives you the wished form and doesnt care for the HD of the original creature it duplicates. (exept the HD cap that is necessary for balance of course)
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What do you think? Please no rants about how overpowered polymorph is. Just rulewise explanations!

 

[Death]

I think you can not polymorph into a special specimen of a species, just the average. You can polymorph into a human, but you can not polymorph into a human weight lifter. Dragons for instance grow in size and HD with age. Average large dragon has that many HD, so that's why I would not allow to polymorph to weaker specimen of the bigger species.

 

[Simulacrum]

Yes I know, You don't get stronger after all, but the Dragon is a bit exeptional. Look, if you transform into a juvenile dragon you already are an avarage large dragon....you're just weaker than an actual one. (less HD)
Still I'll think of it the same way. If there was a form that was Large and had lesser strength then the wizard casting but would have more HD what then?
The strength lifter argument was written into the rules to make clear you could not just pump up your strength by saying you polymorph into Grabu the Ogre Barbarian King with Strength:34!
So the question isn't answered yet!

 

[Simulacrum]

Ok, I thought about it a bit.
A new example: Zuhra the powerful Hagwitch has 20HD
She has of course an Ogre servant and she used to polymorph him into something *huge* to scare off intruders from her cave.
That is interesting because she could transform the Ogre into a huge Dragon, it would be possible from the size category.
This is the crucial argument I searched, not only the 15 HD cap does prevent her from choosing such a form, also gamebalance would be greatly in danger if that was possible. (thús my question)
It would mean that if the Hag had dominated a huge creature she could transform it into an gargantuan dragon...etc.
I think that should work as a good explanation.