Am I missing something?
what I mean is that 4:3 monitors are almost square, and the game runs at a widescreen resolution. I assume it's 16:9 since that's the most common. So what happens is the following:
At its native resolution, Minecraft has these proportions inside our monitor. The monitor's proportions being black and Minecraft being the white box.

If you were to scale the Minecraft window up as high as it goes, you end up with nothingness at the top and bottom of the screen, just like when you're watching a widescreen movie on a non-widescreen.. screen.

But in true fullscreen, Minecraft doesn't do this. Instead, it keeps scaling the picture up uniformly until the black bars on top and bottom are removed. The problem is that this method also crops off a fair amount of information off the left and right sides of the screen, thus our field of vision is noticably narrower when playing in fullscreen because of our almost-square 4:3 monitors. See?
tl;dr MASSIVE field of view loss
an easy fix for this would be to either fix this issue by having it not crop the field of view OR just make the game run a little bit bigger as a default and set the same field of view to it but keeping the same aspect ratio