The bit about comps is sort of true. A compensator works most effectively when there's enough gas generated by the cartridge to run through the comp chambers. Usually this is associated with higher velocities because, typically, more powder means more velocity. Competitive shooters will look for "slow" powders (i.e. formulations that require a higher volume of powder to achieve similar velocities) because of the tradeoff between increased velocity and more gas.
However, competitive shooters also have to contend with maximum magazine sizes, and having smaller calibers (like 9mm) mean that you can fit more bullets into a single magazine than you can with, say, 45ACP, but you also have less internal case volume. 45ACP doesn't really have that issue. It's a big case with a ton of room for powder, pushing a big slow bullet. The more powder in the cartridge, the more gas you put through the barrel, and the better the comp works. Even better if you can shoot light bullets.
I hope this makes sense.
