The guy was huge, but a vegetarian. Where did he get his protein (other than sperm)?
They didn't have protein powder back in his day.
Yes, they did. Soy protein was avaiable, although from all accounts, it tasted terrible.
And, as others have mentioned, Rheo Blair made a milk-and-egg protein powder that bodybuilders back his Pearl's day used.
Pearl was a lacto/ovo vegetarian, not a vegan (someone who consume NO animal products whatsoever). It's not that hard to put on size on a lacto-ovo vegetarian diet.
Nowadays, it's really not that hard to do so on a vegan diet. There are more pure veggie proteins available, than there were then. And, more dietary information is handy, regarding how to combine veggies and grains to get complete proteins.