Change shape is almost entirely cosmetic,
A creature with this special quality has the ability to assume the appearance of a specific creature or type of creature (usually a humanoid), but retains most of its own physical qualities.
(Change Shape special ability description)
so an HD cap would be somewhat odd. Indeed, the only things that actually change beyond your appearance are
[…] It gains the size of its new form.
The creature loses the natural weapons and movement modes of its original form, as well as any extraordinary special attacks of its original form not derived from class levels (such as the barbarian’s rage class feature).
The creature gains the natural weapons, movement modes, and extraordinary special attacks of its new form.
Everything else in the description is about how things don’t change. Combine that with the fact that change shape is find (near?) exclusively on monsters, not player-focused material,¹ and is a supernatural ability, so players cannot easily access it by via their own polymorph effects,² and there just wasn’t a lot of reason to limit it.
So yes, an ogre mage can look like as powerful a giant as they can think of, as long as it isn’t too big. It won’t have much of what made that giant so powerful, though.
Yes, technically, an ogre mage or succubus is legal as a player character so long as you’re playing at a high enough level. But the entire point of the Level Adjustment system is to account for powers that may be more dangerous in player hands than DM hands. And in practice, it goes much too far, basically crippling characters with LA—the ogre mage and succubus aren’t really playable.
It’s a consistent thing that most forms of polymorph don’t allow you to gain the supernatural effects of the new form—that feature was originally limited to the 9th-level shapechange spell. Of course, they later printed Assume Supernatural Ability and Metamorphic Transfer, which make it much, much easier, but those wouldn’t have been assumed at the time change shape was written up—and those are widely regarded as mistakes anyway.