Yep, you’ve hit on the issue with many lines of wargames minis. They are not really sculpted to an actual, consistent scale ratio. You can say a 20mm figure is 1/72 scale, or a 6mm figure is 1/285 scale– but they are not retaining the true proportions of a 6 foot tall man shrunk down 72 times smaller, or 285 times smaller, etc.
Your example of the “3 foot wide shoulders” is just the beginning. Most of us also don’t have pumpkin-sized heads or 12 inch long fingers, but minis are sculpted that way. Partly for durability, but also for visual impact. When you start with figures like that, applying a strict scale ratio for cars and buildings will result in the sorts of situations you show in your pictures.