I suspect some misleading models or missing information. Patties were 10:1 ("ten-one", ten patties per pound) in 1985, or 4:1 ("four-one", four patties per pound) for the Quarter-Pounder burgers, before cooking. And they were all frozen then. So ostensibly there is no size difference before cooking.

It's possible that McD's have changed the fat:lean ratio which could account for greater shrinkage when cooking. Or alterations in bun sizes.