The study, published in the current online issue of the Journal of Evolutionary Biology, found that larger males were better at mating with, and impregnating, females when they competed directly with smaller males.

But when the faster-maturing smaller males were given a one-day head start, their paternity rate was 10 times higher than larger males'.

Mating can last anywhere from six to 31 minutes, but there's a downside for males, no matter their size: they are usually injured or killed by the female during the act.

Courtship between redbacks lasts an average of 50 minutes when males are competing for female attention, and 412 hours for single males.

University of NSW postdoctoral fellow Michael Kasumovic, who co-wrote the study with Maydianne Andrade of the University of Toronto, said the results showed nature favoured larger and smaller males under different circumstances.

''Larger males experience a longer maturation process so they are unable to search for and mate with females and produce offspring at the same rate as smaller redback spiders,'' Dr Kasumovic said.

''Large size and weaponry are strong predictors of a male's competitive strengths because those traits help them dominate smaller males when they compete for food and mating rights.

''However ... smaller males develop sooner than larger males and often mate before larger competing males arrive on the scene.

''Size isn't the only ruler by which we can measure a male's quality.'' AAP