Rhenny
Adventurer
It's not so much that they see a CR 2 brute, as they see the reality which its statistics reflect. They see the huge muscles, which we see as Strength 19. They see the hide armor it wears, which we see as AC 11. They see its size and relative lack of skill, which we see as 59 Hit Points.
Unless it has different stats, in which case they see a different reality. If it has more HP, then they can see that it's bigger and/or more skilled. If it's wearing plate armor, then that's also obvious. Or if it's some homebrew rock giant that I introduce because I think ogres need to be 30% harder, then they can see that it's proportionally stronger and tougher and more skilled or whatever.
I know you don't have to play that way, but honestly, I really don't want to play the game of hiding information from players or making them second-guess what they think their characters should know. It's one of the reasons I'm so dead-set against unnecessary abstraction.
Totally! This is one of the reasons I'm glad there are no scaling bonuses automatically tied to AC.
That said, I do like to alter creatures, but I do make sure that the description I give of them matches their increased prowess (like some will be bigger, stronger, faster, more agile, etc.).