The sign in your photo says Small, Medium and Large, but it doesn't give any diameters, so it can't show the gap by itself. Here is what I can say from memory, without a source in hand.
Most US pizza sizes were standardized around the same few diameters for decades: 10, 12, 14 and 16 inches for a large. Pizza chains of the 1980s mostly sold 12-inch and 14-inch pies as their big sizes. What has changed more is the amount of dough, sauce and cheese on each size, and how the chains name and price their sizes. Many modern chains sell a larger "large" with more topping, and some have added extra-large options, so the top size can look bigger than it did then.
The honest answer is that I don't have a dated study comparing 1980s and today's sizes across chains. Menu archives would settle it. Old menus, Wayback Machine captures of chain websites, and newspaper ads from the 1980s would show the actual diameters and prices.
If you're at a place with a sign like this, the diameters are often printed on the menu or the box, and comparing them with a 1980s menu would show the change directly.
