The title is basically the question. I'm prepping a session, which might feature a battle between the PCs aiding a wounded Copper Dragon and a Red Dragon with its Kobold tribe, so I was looking over the two creatures. But I'm confused as to why both of these Dragons have (almost) the same challenge rating at the same size.
However, the Red Dragon is Large vs the Medium Copper Dragon, has much more strength, better saves, more powerful attacks (because of a higher BAB, because due to its size it gets 3 extra natural weapons, because it has Reach on its bite and because all of them deal more base damage), more HD (and thus much more HP) and a vastly more powerful Breath Weapon.
Why are both of these creatures CR 7? If you pit them against each other, the Red Dragon would absolutely slaughter the Copper one (which, incidentally, is exactly what happened in the story, but still) so what makes it that these two creatures are considered an equal challenge to a party of adventurers?
I'm guessing (at least) one of these two CR values are wildly off the mark, but which is it? Is the Young Red Dragon a reasonable challenge for a 7th level party and the Young Copper Dragon a pushover, or is the Young Copper Dragon a fair challenge and the Young Red Dragon a certain way to wipe your party? I want the encounter to be hard, but I don't want to massacre the party.