Abstract Comparisons between crowded and uncrowded Kochia scoparia individuals demonstrate pronounced effects of competition on plant allometry as well as on the distributions of different aspects of size. Non-destructive measurements of height and stem diameter and, for a subset of the populations, the number and length of leaves and branches, were taken at three times, and the plants were harvested after the third measurement. The sequential measurements afforded the opportunity to obtain information of the effects of competition on allometric growth trajectories of individuals, as well as on static inter-individual allometric relationships. The distributions of most size measures appeared to be normal for the uncrowded population. Crowded populations developed a negatively-skewed height distribution and a high-inequality mass distribution, whereas the diameter distributions remained normal. Plants grown without neighbours showed simple allometric relationships between height, diameter and weight. For isolated plants, the 'static' allometric relationship between plants of different sizes and the allometric growth trajectory of individuals were similar. Crowded populations showed complex allometry; the static inter-individual relationships between height, diameter and weight were curvilinear (on log-log scale). There were large differences in the allometric growth slopes of uncrowded vs. crowded plants. Allometric relationships between stem diameter and plant mass, and between total length of leaves and total length of branches, did not seem to be altered by competition. The data suggest that height was the most important aspect of size influencing future growth of individuals in the crowded population. Only plants above a certain height were able to continue to grow from the second to third measurement in the crowded population. This supports the hypothesis that asymmetric competition for light is the cause of the allometric changes and of the increase in size variability due to competition.