WASHINGTON—Two leading experts on the “gig economy” now say their estimates of its impact were too high, skewed by spotty data and the recession of a decade ago.

Alan Krueger of Princeton University and Lawrence Katz of Harvard sifted through new evidence to explain how, in a 2015 survey, they overestimated how people cobbling together a living from odd jobs, especially via apps like Uber, would upend traditional work arrangements.

Earlier this decade, many researchers and journalists fretted that the gig economy was taking over the way people work. When the Labor Department finally studied the question in detailed research released last summer, they concluded the gig economy had scarcely changed the U.S. labor market.

Messrs. Krueger and Katz now explain how the thesis itself was flawed, in a working paper to be released this week.

First, the gig economy appeared swollen largely because the labor market earlier this decade was so weak for so long in the aftermath of the recession. Rather than heralding a permanent shift in the relationship of Americans to employers, a lot of gig-economy activity was odd jobs that people took up to make ends meet. As the economy returned to normal, they returned to more familiar work arrangements.