Research at St Andrews University has established that size does affect sexual performance - in seed bugs.
The study found that a male insect had less reproductive success when its penis was cut short.
There is a known link between penis length and successful reproduction in some animal species but it has rarely been tested experimentally.
The St Andrews' biologists examined a species of male seed bug with a coiled penis which is 70% of its body length.
Writing in the Proceedings of the Royal Society B, they said they shortened the organ, then used X-ray scanning to create high-resolution 3D models of the mating process.
This showed that the shorter the penis was cut, the less the reproductive success.
The paper said the size reduction was achieved using scissors and without harming the bug.