Could be that the hands you use to hold the Big Mac are a bit bigger than they were back in the day.
I'm not sticking up for McD's. My problem with them is that their hamburgers seem to not be made of meat.
According to my stomach, their sandwiches are made from fat that's been scientifically textured to look like food. Sort of like CGI you can chew on.
It’s actually weirder than that. While the meat is from beef steers, it is heavily processed, more like pink slime, and the fat is added from dairy cows. In processing, while the meat is being ground and reground, they add big slabs of fat to get the exact fat content they want. Then the only other ingredients are salt and pepper.
One rumor about adulteration was that they added earthworms, not only false and McD’s has no sense of humor about it and sues; but as someone pointed out, earthworms are much more expensive than beef. A burger made of worms would probably cost $15 or more for just a single patty.