Back in the late 80s, they sometimes did make short trips with big planes. For instance, when I was interviewing for jobs, I had to fly from Cleveland to Detroit and it was in a huge plane. I think it was a 767, or maybe the larger variant of the 757. It was about 25% full. I suspect that they wanted the plane in Detroit thus why they used it because it was only about an hour flight.
That was about 4 years before Home Alone 2 was filmed.
I agree with you though. Flying in the 80s and 90s was different than flying now. They definitely pack people in as the seats are less roomy and I rarely encounter a flight that isn't full.
They've also kept increasing fees for things to the point where we feel they are "normal" now.
The difference is night and day from the 80’s until now. Can you imagine People’s Expression existing now with paying for your ticket on the plane while in mid flight?