Part of it has to do with effort levels. Pitchers with longer limbs can create more leverage, while the smaller pitchers have to resort to higher-effort deliveries to compensate.

I asked Corey Dawkins for more insight on how the workload affects different body types. Here's what he said:

Weight loss over the course of the season does not have as much of an effect s how quickly it occurs, although certainly there is a point where there is a negative effect even if done gradually. The smaller players have less to give, so to speak, but it's extremely rare for a major league pitcher to get to the negative zone.

Velocity and effort also have a significant role. More of the shorter/lighter pitchers are not throwing the ball over 95 mph, although there are always exceptions. Throwing at maximum effort increases the forces placed on the structures of the shoulder and elbow, which can lead to injury over time.

We tend to see the most pitching injuries in spring training and the first few months of the season, leading to the velocity of weight loss being the issue and not the number of pounds.

Hopefully that helps.