It's about even pressure along the span of the wheel when it's all touching the ground. Here, there's going to be two points of pressure, because most of the wheel between the ball and the its far side isn't touching the ground. So about half the weight on that wheel will go through the golf ball into the ground (the first point of pressure), and the other half will be applied on the ground by the far side of the wheel (the other point of pressure). Assuming the wheel is supporting half the weight of the steamroller, about a quarter of the total weight of the steamroller is applied to the ball. So say the steamroller weighs 20,000 pounds, the golf ball js being squished by about 5000 pounds.

Putting the golf ball in the middle of the wheel would put most, if not all of the force applied through that one wheel through it. So that would double the force that's squishing it, to about half the total weight of the steam roller. About 10,000 pounds squishing the gold ball for the same hypothetical steamroller.