Source: Publishers Weekly, Jun 2012
The vigor and depth by which the man thrusts his penis into the vaginal canal results in more effective semen-removal (so size does matter in this sense, for the man as well).
The more intense the arousal, the more powerful the ejaculation and the deeper it lands in the vaginal canal, making it more difficult for any males that follow him to retract his sperm with their penises.
Once ejaculation has occurred, men typically become flaccid fairly quickly, and further stimulation of the penis is even uncomfortable. This is important because continued thrusting would be self-defeating: the man would essentially be removing his own sperm at that point.