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Seeds, growth rates and duration of growth influence plant development. However, we lack a mechanistic understanding of how they lead to larger and higher‐yielding plants, as these traits have not yet been explicitly studied in combination and across ontogeny. Seed size and growth dynamics have evolved differently during domestication and improvement. Nevertheless, it remains unclear whether the relationships between these traits and their contribution to plant size and yield have also changed over the course of crop evolution.Here we grew wild, landrace and improved accessions of 18 annual herbaceous crops in a glasshouse. For each plant, we measured seed mass, growth rate and duration of vegetative growth. We also measured plant size at three ontogenetic stages: seedling, juvenile and mature, and reproductive output. Using path analyses, we tested causal relationships between the traits and quantified their relative importance in determining mature plant size and yield.Seed mass and duration of vegetative growth were more important than growth rates in explaining variations in mature plant size and yield among species. Domesticated plants were larger, had heavier seeds and higher yields, but did not grow faster or for longer time‐spans than their wild progenitors. Trait relationships did not differ between the wild, landrace and improved accessions.Our results suggest that annual herbs reach larger sizes primarily through a combination of heavier seeds and longer vegetative growth periods. Moreover, domestication has increased plant size only through the heavy‐seed causal pathway, via cascading effects during ontogeny. However, the high yields of modern crops hardly be explained by the traits considered here, suggesting the importance of other drivers, such as roots and their microbiome.Synthesis. We provide a better mechanistic understanding of the size axis of global plant trait variation and emphasise the role of growth duration in explaining the diversity of mature plant sizes. Seeds and growth dynamics are highly functionally coordinated with plant size, and this coordination has changed little during crop evolution. Our results highlight that multi‐trait relationships throughout plant ontogeny play a key role in governing how domestication has influenced plant size and crop yields. [ABSTRACT FROM AUTHOR]