It is way easier to grow muscles than growing the penis, expecially with steroids (some pornstars are obvious steroids users I think). Implants and injected substance are not uncommon at all in the world of bodybuidling
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and it’s not unlikely that there is even a larger percentage between people who don’t compete in any sport. Pumps are pretty common among porn actors I have read; some porn actors are clearly overpumpers.
In the end, you can do both, and if it is a matter of job, why not doing both? But some actors have done something not natural to their penis. Another tha comes to mind is Erik Everhard.