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Abstract: The theory of life‐history evolution investigates how growth‐reproduction trade‐offs drive evolution of body size in uni‐ and multivoltine (one or more generations per year) arthropods. Existing theory does not predict how the length of the feeding season (season length hereafter) affects body size in semivoltine (i.e., juvenile period longer than 1 year) determinate growers and usually ignores that uni‐ and semivoltine arthropods accumulate large reserves to cover costs of diapause and future reproduction. Here, we present how the trade‐offs between growth, storage and reproduction drive evolution of body mass and reproductive strategy in arthropods with determinate growth. Our life‐history model concerns high‐latitude marine copepods living in a strongly seasonal environment. We find that small changes in season length and mortality rate translate into abrupt shifts in lean body mass (a proxy for body size). Body size shifts are caused by a change from multi‐ to uni‐ and semivoltine life cycles with semivoltine life histories selected for in short seasons and only if background mortality is low. Shifts in the number of generations per year do not translate into shifts in the mass of lipid reserves. The model predicts less reserves the shorter the winter. Season length alone is not a sufficient predictor of the degree of capital breeding. Storing for reproduction is strongly selected for under short season but low mortality rate. Hence, capital breeding contributes to fitness in uni‐ and semivoltine organisms whereas multivoltines are income breeders. We also show that storing reserves for diapause and capital breeding trades off with adult size of determinately growing arthropods. Our results, in particular regarding optimal body size, reproductive strategy (income‐to‐capital breeding) and degree of storage are relevant to a number of determinate growers, including insects and crustaceans. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]