Birth weights have increased by a pound and a half and lengths by half-an-inch in recent decades.
And although scientists can't be sure what has caused the changes, it may be because the average size of mothers has increased.
The research, published in The Journal of Pediatrics, used data going back to 1929 to track babies' sizes at birth and beyond, and found that those born after 1970 were heavier and longer than babies born in earlier decades.
"What would have been considered a big kid in the 1930s would not have been considered a big kid today," said Ellen Demerath, one of the study's authors and an associate professor in the University of Minnesota.
But by age one, most babies were about the same size as counterparts in previous generations, suggesting that babies born smaller in the past experienced faster catch-up growth in their first year of life to arrive at similar average weights as the modern infants.
The researchers studied 620 babies from birth to age three, and all were of European ancestry.
"These are your middle-class, white, semi-urban, suburban dwellers that have been tracked," said Ms Demerath. "And there have been huge changes in infant growth."
The average size of mothers, however, has definitely risen in recent decades, as gauged by body mass index (BMI), a measure of weight relative to height.
Between 1930 and 1949, 18 per cent of mothers in the study had BMI's that qualified as "obese", while 48 per cent fell into that category between 1990 and 2008.
Ms Demerath said the main message of the study was that maternal health is in a different situation than it was decades ago.
But, she and her colleagues concluded, growth rates in the first year of life cannot explain trends toward obesity later on.
More:Advice And Health
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