A Burger King Whopper today is the size of a Big Mac of years gone by.
The King still does burgers better than the clown.
Big Macs have not changed size.
Maybe things just seemed bigger when you were a kid, and smaller in comparison.
(A while ago they temporarily had 3 "sizes" of Big Macs but that was a promo.)
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?
Actually easier than either of those, to be honest.
The intent is to segregate price-sensitive customers from convenience-sensitive ones. (The app orders make in-store ordering quicker.)
The problem is there are confounding factors in tech saviness and privacy sensitivity in requiring price-sensitive customers to go through an app. (As well as the basic awareness problem.)