  The formation of planetesimals was a key step in the assemblage of planetary
bodies, yet many aspects of their formation remain poorly constrained. Notably,
the mechanism by which chondrules -- sub-millimetric spheroids that dominate
primitive meteorites -- were incorporated into planetesimals remains poorly
understood. Here we classify and analyze particle-size distributions in various
CO carbonaceous chondrites found in the Atacama Desert. Our results show that
the average circle-equivalent diameters of chondrules define a positive trend
with the petrographic grade, which reflects the progressive role of thermal
metamorphism within the CO parent body. We show that this relationship could
not have been established by thermal metamorphism alone but rather by
aerodynamic sorting during accretion. By modeling the self-gravitational
contraction of clumps of chondrules, we show that (i) the accretion of the CO
parent body(ies) would have generated a gradual change of chondrule size with
depth in the parent body, with larger chondrules being more centrally
concentrated than smaller ones, and (ii) any subsequent growth by pebble
accretion would have been insignificant. These findings give substantial
support to the view that planetesimals formed via gravitational collapse.
