This has been discussed here before: however, in my opinion this trait is favored more by social environs than those found ‘in the wild’. For example, if there was any advantage conferred by a large dong in the latter (wild), then surely other primates such as gorillas would’ve evolved it by now. Our species, however, has the largest penis of any of the primates, including apes who often weigh 3 - 4 times as much as us.
As our semi-mastery of the wild environment has progressed, the environment that more and more tends to shape sexual selection is the social environ: this social sexual selection is responsible for refining traits already inherited from the wild, including penile size.
Others have suggested a larger penis is also correlated with a higher likelihood of conception due to being able to deliver sperm closer to uterine point of conception, but I know of no facts that support this.