The thickness of the tubing is dependent on what sort of alloy is used. Think, cheap tubing is not as strong, so it'll be thicker to achieve the same strength. High-end stems (and frames...) use top-level alloys and can be made thinner as a result.
You can figure that no manufacturer is going to sell a stem that's likely to fail, and thus be the source of an expensive lawsuit.
Most of my bike officers are big, heavy guys who wear a lot of heavy equipment as well. I've never seen a broken seatpost.