Let's look at a different size difference now... But let us take a pound of bacon, which was enough to feed a family, and have one or two pieces left over, now a pound of bacon if lucky is 10 slices, at the same price or more for a pound of bacon...
Not sure what you're getting at here. A pound remains a pound. Perhaps the slices are thicker, but uncooked package weight remains the same.
When my mother made bacon we each got a couple of slices, not six or more. Bacon was a side dish, not the main meal. I did the same. DH, kids and I were a family of five. When I served bacon I cooked a single pound only.
Big Macs likely appear smaller to people because they're dwarfed by huge burgers served at other places. It's not the Big Macs that have changed, it's our expectation how big a burger should be. Thus the obesity crisis.
Big Macs have 590 calories, and a Mac Combo Meal (medium fries & medium soda) comes in at 1120 calories, nearly a full day's worth of calories for most seniors, 64% of which is fat (49 g).
According to a Mayo Clinic calculator, for my age, size and activity level, my daily calorie intake should be 1550-1700.
When I worked at McDs in the early 70s, Big Macs were mostly purchased by teenage boys, young men and construction workers. Children virtually never got them.
According to numerous sources, Big Mac Meals cost $5.99 in the US.
https://www.mayoclinic.org/healthy-lifestyle/weight-loss/in-depth/calorie-calculator/itt-20402304
https://www.mcdonalds.com/us/en-us/meal/big-mac-meal.html#accordion-c921f9207b-item-842cb18782