big Mac is the burger patty. The regular burger patty at McDonald's is almost inedible. I try not to be too hyperbolic but it really is, objectively, bad.
I like Mac sauce, though, and the bun and everything else is fine, too.
So, while it would be an absolute calorie explosion, the real way to make a BIG Mac would be to put the quarterpounder patties on it. While the QP patties at McDs won't win any awards, it is at least more akin to an actual, real life burger patty than the regular McD's patty.
If I do eat McD's (and I'm not getting a cheap Hot and Spicy) I already get a QP with Mac sauce.
Mac sauce haters can move on from this thread. It just isn't for you.