I mean, they haven't.

I don't know why you think they have, but they haven't. You're mistaken. You've noticed wrong.

Maybe you came across a claim about it on social media, but it's been entirely debunked:

https://www.snopes.com/fact-check/big-mac-since-1980/

And Whoppers have nothing to do with anything here. I don't know why you're comparing the two -- the Whopper is a single-patty quarter pounder, and it compares to the McD's quarter pounder. Not the Big Mac with its double patties and extra bread.