Here’s an original iPhone (3.5″ display) next to my iPhone 12 (6.1″ display). And keep in mind, the Plus/Max phones are 6.7″ displays:
If you look, you can see that the display is even tinier, thanks to the chonky bezels on the top and bottom. I still kind of miss the home button though (or more to the point, Touch ID, which was later added to it). And when I hold the original iPhone in my hand, my thumb can easily cover the entire screen without any straining, which is nice. I can more or less do the same on my iPhone 12, but it involves stretching and the phone shifts somewhat precariously as I move my thumb around the display. It’s not really meant to be a one-handed device.
While the original is definitely not as wide as the 12, the change in height is far more dramatic. This makes sense, since we have not evolved wider hands in the last decade.
It was fun to hold an original iPhone (I had an iPhone 4 in 2010 and while it had flat sides, its dimensions were pretty much the same), now I just need to find my one surviving 30-pin cable to see if it will power up (I do not have huge hopes for this).