The game was always designed to run in 4:3 ratio, which all monitors were back in the days of WeMade. Modern monitors are typically 16:9 (widescreen) ratio, which gives us 3 options
1. Run in window mode
2. Run full screen but stretched (what you have now)
3. Run full screen at correct aspect ratio, which leaves big black "bars" either side, because the game takes up the full height of the monitor but not the full width
To switch between option 2 and option 3, some graphics cards simply don't allow it, but you can try changing your screen resolution to a resolution that is 4:3 ratio such as 1024x768 then trying the game again