That's because -fov 0.75 in Freespace isn't the same as setting FOV to 75 in other games. Most games have a direct correlation between their FOV settting and the actual Field of View in degrees. In Freespace the default 0.75 FOV is actually something else. It's the vertical angle of view, in radians, multiplied by 1.39626348. for some reason.
First, you'd have to convert 0.75 radians to degrees which would be ~43 degrees.
Next you multiply that by 1.39626348 to get ~60 AoV. How this converts to FoV depends on your monitor's aspect ratio, but let's assume you're using a 16:9 widescreen one. Multiply 60 by (16/9) and you get 106.666...
The default Freespace FOV for a 16:9 monitor is actually ~107 degrees, which is quite high and this is why it causes "fisheye" distortions and makes big ships less impressive.
For a 16:9 monitor you need to divide whatever FOV you actually want by 142.2222 to get the number you need to use with the -fov flag. So if you want a 90 degree FOV you need to use -fov 0.6328.
On an old school 4:3 monitor the default FoV would be only 80, something that's much more natural.
Also you couldn't fit a full sized Orion into HL2, it's too big for the Hammer editor. The largest hammer can do is ~0.8km in height, width, and lenght while the Orion is 2030m long. You could obviously fit the model in but you couldn't make it big enough to illustrate the difference between the player , a tree, and that thing.