Big Macs have gotten perceptibly smaller while Whoppers have stayed the same. Maybe the weight is still the same (many ways to play that game) or whatever but there are tons of people who have noticed the shrinkage.
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.
Do you disagree with Snopes? Do you think they're lying to you?