In my experience, pitchers (particularly skinny pitchers) typically gain velocity when they gain weight*. (I, for one, gained 60 lbs and about 7 mph on my fastball over my college baseball career and I have helped a number of skinny guys do the same.)
That being said, chubby guys have no business gaining more weight (and fat) – these athletes are better off focusing on food quality rather than quantity and gradually decreasing the contribution of fat mass to total body weight and increasing the contribution of lean tissue (i.e. muscle, bone, connective tissue, etc.)
While rapid or excessive weight loss is definitely not advised, pitchers with a higher body fat % will likely experience improved performance by getting leaner – as @Steven_Ellis alluded to, most pitchers should sit between 12-14% body fat.
** Correlation doesn’t necessarily equal causation and there are other important factors at play!*