actually there is some correlation to dick size versus erectile strength that diminishes as we age. i made a friend on this site who had a bonafide 13.5" by 6" dick and he said that he started losing maximum erectile strength in his early twenties (he was in his early thirties at the time). his problem had NOTHING to do with hormone deficiencies, diet, blood pressure or anything else that his urologist could think of as a cause.
taking viagra did NOT help that much (he got somewhat harder), BUT after he tried the jelqing program for about 30 days (along with the male supplements that made him hornier so he would be more inclined to keep up the program), his penis returned to its FULL erection stength to the delight of he and his wife.
this complaint is NOT all that uncommon for those who are truly endowed. i can't explan why, but my guess is the blood volume of the corpora cavernosa and the amount of pressure needed to acheive a full erection is the same but for various reasons this mechanism gets congested and/or deteriorates quicker in those that are very large or maybe to due quick growth during puberty, the maturation process stopped before the internal mechanisms needed to support this size reached their optimal development which maybe wont become apparent until the twenties, thirties or later. this problem was stated and also addressed MORE often in the previous incarnations of this site.
as far as doctors being the absolute gurus in this area, they are sadly lacking. and even if jelquing WAS the "prescribed" remedy, they would probably NOT be inclined to recommend it, since their training is mostly in prescribing DRUGS.