It may be worth considering that a heavier person needs a stronger heart than a lighter one. The heavier weight also acts as a constant load/training. Without some degree normalization we won‘t know whether this is normal or concerning.

This is a very thoroughly studied phenomenon. The hearts of obese people are generally more muscular as you say, but not in a good way, so I wouldn't compare this to training. In overweight people, the heart walls get thicker and the volume of blood that the heart pushes out with each stroke is decreased as a result. This means their heart needs to beat faster to reach the right throughput and their heart is under constant strain, kind of like having your car overrevved at all times.

With exercise, the heart muscles grow in a different way, and the volume of blood contained inside is not reduced. So without looking at the heart itself, we can't even tell whether a lot of muscle is good or bad, we also need to look at the rest of the context.