Do you have any scientific evidence to validate the diagram you posted?
I am very skeptical (a nice way of saying this is bullshit). Valid scientific study into the penis size show the average is about 5 inches. That is from a peer reviewed study where the researcher measure the penis as oppose to man reporting and boasting the number.
http://www.jurology.com/article/S0022-5347(01)65682-9/abstract (this one you need to access through library)
So most man can't satisfy the woman?
That can't be true.
From an evolutionary point of view, orgasm is a reward to encourage mating to ensure members of the same specie mate and the penis and the vagina is like a key and lock system (yes, some closely related species of animals actually have different shape reproduction organ to mechanically avoid accidental mating that would be evolutionarily detrimental).
In a surviving specie, the penis must fit into the vagina. New MRI technique allow for very accurate measurement of the female reproduction organ. A study by Barnhart et al. (2006) shows that the length from cervix to introitus is 62.7mm, with the width at the introitus at 26.2mm. So if you approximate the width as the diameter of a cylinder, the circumference is about 164.62mm (about 3.25 inches).
humrep.oxfordjournals.org/content/21/6/1618.full.pdf+html (free access)
So 3 inches in length and 3.5 inches in diameter will be minimum sufficient. Something bigger like in the 4-6 inch range (which is the validated average of human penis size) would be anatomically ideal.
Who ever make that up probably didn't know anything on reproductive science. Another reason for this can be because this is just a popular science survey, so no actual measurement was made and the woman just make a number up based on visual stimulant effect (a big penis is more exciting to look at than a smaller one).