Couture has some of the best wrestling of any MMA fighter. It allowed him to survive against Lesnar. He was still losing the fight in all regards except possibly octagon control. He just wasn't getting his ass whooped.
Someone his size without amazing wrestling is going to get beat down by Lesnar. There is only one other fighter that has really good wrestling in the HW division and you think he is garbage...
Mir can work on his wrestling and it will help him but he isn't going to get to a level even close to those fighters. Bulking up and increasing his strength is his best and most readily available tool in giving him a chance of winning the title back.
There's a lot of wrestlers at HW. If you're talking about Velasquez... that dude is probably the worst match for Brock. He's like a mini Brock.
Nog has better wrestling than Couture (as he showed when he threw Randy around), and I think he'd stand a much better chance against Lesnar. And it has nothing to do with his size.
Fedor would beat Brock but not because he's bigger. JDS will likely beat Brock in the future and it won't be because of his size.
If Mir beats him again I doubt it will be because of 20lbs, it'll likely be because of a much improved TDD, a way better game plan, and improved striking.