Looking at pics of the older big macs, it appears that the bun has gotten smaller, but the size of the meat patty remains the same as it always has been: two 1.6 oz patties. It's the same size patty that goes on a McDonald's cheeseburger. It's entirely possible that the fat content of the beef has changed over the years--higher fat beef is cheaper and will cause more shrinkage upon cooking.
McDonalds has actually increased the size of the quarter pound patty. Now instead of 4 oz (1/4 lb) pre-cook weight, it's 4.25 oz, because of complaints about how the patty shrinks when cooking.