They wouldn't, and that's not what I have a problem with. A hill giant should be a hill giant. It shouldn't be a level 10 brute when attacking a village of commoners and suddenly transforms to a level 10 minion when fighting adventurers. I also have a problem with something ten ft tall weighing 1000 or so pounds having 1 hit point. You may not have a problem with this, and that's fine. It just doesn't suit my tastes.
It doesn't transform, and it doesn't have "1 hit point."
When attacking the village, it's essentially without stats. It exists as a narrative function, unless you're going to seriously grab your dice, stat out the village, and actually roll that combat in your free time, and I'll assume most people will not.
Stats are based on when and how the are used - otherwise they wouldn't exist in the first place. A monster's stats purely in limbo are meaningless. What matters is how they reflect or act on the game itself. Stats by their very nature are an artificial game construct.
Everything in D&D is a narrative construct - an "item" that exists to interact or be interacted with in a different set of systems. A hostile creature is a narrative construct that is meant to be interacted with in some sort of encounter, be it sneaking past them, fast talking and fooling them, or fighting them. The minion is the same type of narrative construct, with the difference that they represent a very minor threat (while still representing a threat - again, stats exist to be
used ) rather then a moderate one.
Combat is the same. In every edition, from Gygax's first notes, HP has been an abstraction of narrative - the abstraction of "health." It has never been pure physical well being; after all, we're surviving being "hit" by a sword. Combat, equally, is an abstraction - you aren't just full attacking and then standing rigid like a chess piece, you're dodging and weaving and blocking as the fight goes on. Again, the enemy survives despite us scoring a "hit" on them. The "hit" and "HP" and "AC" and all of that are abstractions we use because frankly a super realistic combat game would be far, far too much of a hassle. So the orc takes one "hit" at level 10 but three "hits" at level 1, yet we can, I think, agree that it's absurd for anything to require more then one "hit" when you're supposedly running them trough.
The minion is little different - the only difference is that one monster needs several "hits" that can come from multiple sources while another needs one that has to be a direct "hit."