Ever see a huge bodybuilder get out-lifted by a smaller powerlifter? Yup.
Why? The bodybuilder rarely trains under maximal loads, while the powerlifter trains to be strong for those complex movement patterns.
Strength is very much a skill.
Just like in order to get better at hitting a baseball, you have to practice swinging a bat, if you want to get stronger you have to actually lift loads that challenge your strength. Principle of specificity 101.
Strength is first and foremost a neural adaptation and a skill. An individual will get stronger relative to their baseline strength as a result of repeated practice lifting heavy weights.
Squatting, benching, deadlifting, overhead pressing etc. for a 1-rep max involves complex movement patterns, with multiple muscle and joints working together to produce a force. The more an individual can practice specific movements patterns under a specific load the more effective the body becomes at recruiting all neurons and muscle fibers required for a given movement aka more strength gains.
Anatomy is another factor involved in how heavy they can lift weights. Muscle architecture, moment arms, and fiber types all can influence strength. The body is a complex unit, with multiple systems working together.
While I don’t believe muscle size is correlated directly to muscle strength, I do believe at some point, a greater amount of muscle volume, and thus, more fibers per unit of muscle mass will result in a greater ability to exert force positively contributing to strength And therefore, powerlifters may benefit from taking some time to focus on hypertrophy.
If your primary goal is hypertrophy, training in the 5-20 rep ranges (and mostly in the 8-15 range) is “best” in my opinion.
As bodybuilders, we want to focus on effective reps, overloading the (target) muscles with mechanical tension with the intent to grow.
We’re still lifting heavy relative to the rep range a weight that gets you to at least an RPE 8 (RIR 2) within those hypertrophy rep ranges, and getting stronger over time within that rep range.
Keep the goal the goal