Emperor Evulz has lousy sex life because he has a small penis.
Bob's penis is ridiculously large, and when they have sex, Alice gets off so hard she sleeps for days, purring. Alternatively, Bob has multiple penes, all huge. Cue similar reaction from Alice.
Emperor Evulz is a eunuch and no sex, ever, of any kind. Even his random dirty thoughts suck.
Downplayed: Bob has good sex because of his larger than average penis, but he's glad it's not too much bigger.
Bob's partner both can handle and enjoys large members.
Emperor Evulz is strongly against non-penetrative forms of sex.
Bob has a great sex life and a small penis.
Emperor Evulz has a terrible sex life and a huge penis.
Bob tells everyone about how great his and Alice's sex life is, and proudly shows off his large penis. Turns out the sex is great for completely not penis-related matters.
Everyone supposes the problems in Emperor Evulz's intimate life are caused by his small penis. Turns out he actually has a large penis but still has lousy sex life.
Bob thinks Alice likes how it feels, but it actually hurts her because her lady-parts aren't loose enough and as a result, get stretched out during sex or because it is too long and pokes parts of the body it really shouldn't.
Bob's great skills as a lover wouldn't exist if his large penis hadn't attracted troves of lovers to him earlier on.
Emperor Evulz gets his already sizeable penis enlarged, and his sex life gets magically much more satisfying.
A small female population enjoys small penises and spends all of their time lusting after Emperor Evulz's.
Bob claims to have a huge penis and great pleasuring ability in order to impress Alice, but then later admits lying, revealing that he does have a huge penis, but that it's too large to have sex with. He was lying about his list of sexual conquests in the first place. But then Alice turns out to have magical genitals that actually can accomodate Bob the Chosen One's impossibly large penis. However, the sex isn't great from the start given how inexperienced Bob is.
Prince Evulz has his genitals mutilated by his evil girlfriend Queen Marianne, causing him to go insane and turn to The Dark Side. Later on, he has plastic surgery to reconstruct his penis to a ridiculously huge size, only to find out that it's too big to actually use. Insane and obsessed as he is, he keeps insisting his penis size will guarantee him the most pleasure imaginable, so he gets practical and makes orifices suitable, imagining the screams of his victims are a sign of enjoyment.
Penis size is never brought up, or if it is then Bob and Emperor Evulz are both average-sized, while having varyingly pleasing sex lives.
Alternatively, penis sizes may vary but it has no correlation to the quality of sex lives.
Enforced: "I want to portray the male lead as exceedingly virile and attractive, but (I/the viewers) have no idea how female attraction actually works. Aha! I've got it!"
Lampshaded: "He's The Hero. Of course, he has a child's arm between the legs!"
Alice deliberately seduces Bob for his big penis, expecting great sex.
Bob and Emperor Evulz undergo plastic surgery to give themselves average penis size.
Bob finds alternative means to satisfy himself and Alice.
What Evulz lacks in size, he makes up for in sex appeal and skill.
Discussed: "Once you go big, you never go back."
Conversed: "That would hurt."
Bob is unable to have sex because his penis is horrifying and extremely uncomfortable to his partners. Alternatively, Bob feels like he's just being objectified and used like a glorified sex toy.
Emperor Evulz gets so obsessed about the size of his penis he completely ignores all the other ways to improve his sex life and makes the life of his harem miserable.
A young virgin, believing in this trope, decides to pursue a partner with an especially long penis, making her first time a really painful memory, perhaps turning her off from sex for a long time.
Bob has plastic surgery to reduce the size of his penis to a more convenient size, and is able to enjoy sex in a whole new level. Alternatively, he finds a girlfriend who by innate biological features or sheer dimensions is able to comfortably accommodate his penis.
Emperor Evulz realizes that penis size has nothing to do with how well he can pleasure a woman, and gets to it.