Why this research MattersGrain size is potentially yield determining in wheat, controlled by the ubiquitin pathway and negatively regulated by ubiquitin receptor DA1. We analyzed whether increased thousand grain weight in wheatda1mutant is translated into higher grain yield and whether additional carbon provided by elevated (e)CO2can be better used by theda1, displaying higher grain sink strength and size. Yield‐related, biomass, grain quality traits, and grain dimensions were analyzed by two‐factorial mixed‐model analysis, regarding genotype and eCO2.da1increased grain size but reduced spikes and grains per plant, grains per spike, and spikelets per spike, independent of eCO2treatment, leaving total grain yield unchanged. eCO2increased yield and grain number additively and independently ofda1but did not overcome the trade‐off between grain size and number observed forda1. eCO2but notda1impaired grain quality, strongly decreasing concentrations of several macroelement and microelement. In conclusion, intrinsic stimulation of grain sink strength and grain size, achieved byda1, is not benefitting total yield unless trade‐offs between grain size and numbers can be overcome. The results reveal interactions of yield components inda1‐wheat under ambient and eCO2, thereby uncovering limitations enhancing wheat yield potential.