I like to evaluate things based on common sense facts.
There must be some finite limit, some point where the blood taken from your body to fill the penis causes the body’s blood pressure to drop. And with lower blood pressure the erection won’t be as hard, which is a big problem with blood pressure medication.
And like most effects in the body, theres probably a sliding-scale or a “curve” where as the penis volume gets larger the erection hardness reduces.
Getting too big to get good hardness is something I worry about and for that reason (and for women!) I’m limiting girth gains so my erect volume does not get too large.
As an example, the porn stars with long but thinner erections (look great but small volume) seem to get much better erection hardness than the guys with large girth (where the volume gets huge). And the guys with larger bodies (Lex Steele, Napier) seem to get better hardness than the small built guys who have less total blood volume in their body.
One other point, bodybuilding increases your muscle mass and blood volume significantly, so it might help to counter the blood lost into a large erection volume. In porn you see some correlation of the muscular guys having better hardness too.