You are correct...now go to 6000 ft and the barn should be much smaller than whats shown in the game. The size of the barn is like your at several hundred feet.
That doesn't make any sense. If it is correct at 50ft, it will be correct at 5,000ft and 50,000ft. It cannot be correct at some ranges and incorrect at other ranges.
Are you sure you weren't looking at a barn sitting at 2,500ft when you were flying at 6,000ft or such? Ground level is not sea level.