There is zoom in the game.
The most popular monitor size on steam is 20-22 inch these days with a 1680x1050 resolution.
http://store.steampowered.com/hwsurvey
So say 22 inch is the size of said monitor. This is the diagonal width of a monitor with a 1680x1050 resolution or a 16:10 aspect ratio. Which is equal to a 18.6 inch width.
18.6 inch is equal to 0.47 meter. A user on average sits about half a meter or so from his screen.
Filling in the above formula gives you that Alpha is around 25 degrees, meaning that your computer screen takes about 50 degrees of your vision.
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In order to make characters appear at the right size as they would be in real life you would then need to run the game at a 50 degree fov. Since such a fov gives you serious tunnel vision and lacks the near 180 degree fov you have in real live, most games use for instance a fov of 90 (everything appears about 2 times as small as in reality but you can see a bit wider).
TWI decided to by default use a fov of 85, with some additional fov indicator at the side of the screen to simulate the 180 degree fov. And optionally by pressing the breathing key they give you a fov of around 50 so you can see a person as big as they are in reality on your screen. Allowing people to kill at realistic ranges.
So the fundamental basis for choosing it is not unrealistic
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However there are negative aspects as well to using this.
Due to having one real life size (zoomed) version of sight, and a wider peripheral version of sight. You often need to switch between both modes to get a full picture. This means you get a lot of transitioning between a zoomed and unzoomed which can for some people kill their immersion.
And as everything appears about twice as small in your peripheral vision it ends up that you can not spot distant enemies with your peripheral view, which in my opinion makes your peripheral vision pretty useless unless you are in close quarters combat (my experience from arma). As enemies are double as big in zoomed mode they use 4 times as many pixels making them generally 4 times easier to spot, so to spot enemies it forces you to use your "focussed" vision.
Next to that the system seems to use a set fov for the zoom factor (according to the bash and slash irc questions) , so if you sit twice as far from your monitor as an average person, everything will appear twice as small as it would in real life. And of course not everybody uses a 22 inch monitor either as people have monitors from 14 inch laptop screens to 50 inch tv screens or multi monitor setups.
Which is why I personally hope that
within bounds set by twi, players can slightly adjust the fov for the peripheral view, normal ironsight view and the zoomed in view. So users can make the 1:1 zoom work on their home setups.
Something else that could work next to that is rather than basing the bounds of modifying the zoom factors based on an average size of a monitor, basing it on an average dpi resolution of monitors. And then using the actual in game resolution to create an estimation of the monitor size. I made a post about that here.
http://forums.tripwireinteractive.com/showthread.php?t=35969&highlight=arctan
