On average, they have a body length of 49.4 centimetres (19.45 inches), an international study found.
Previously, it was suggested ethnicity was largely responsible for the widespread variation seen in the size of babies.
The research suggests race and ethnicity contribute little to baby size. What matters more is the education, health and nutrition of mothers, and the care they receive during pregnancy.
Overall no more than four per cent of differences in foetal growth and birth size could be attributed to population differences.
Scientists taking part in the Intergrowth-21st study looked at almost 60,000 pregnancies in urban areas of the UK, US, Brazil, China, India, Italy, Kenya and Oman.
Lead researcher Professor Jose Villar, from the Nuffield Department of Obstetrics and Gynaecology at Oxford University, said: "We are not all equal at birth, but we can be. We can create a similar start for all by making sure mothers are well-educated and nourished."
Findings from the study, funded by the Bill and Melinda Gates Foundation, are reported in the journal The Lancet Diabetes and Endocrinology.