Has anyone come across any scientific research or insights explaining why certain pimples turn extremely large, red, and inflamedwhile others stay small and relatively mild?

For instance, I recently noticed four small pimples appear on my shoulder blade. After about three days, two of them became significantly swollen, red, and tender, while the other two barely changed.

I do occasionally sleep on my side, so I initially thought external pressure or friction might be the cause. But that makes me wonderwhy did only two out of the four pimples react this way? Could it be due to how the immune system responds in certain spots, or is something else at play?