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Over the last several decades, cows have gotten bigger due to selection for maximum growth in sires and subsequent maternal traits and heifer retention. During this same period, temperature, drought frequency, and drought severity have also increased. Because cow-size influences maintenance costs, we assessed the effect of cow-size on weaning weight and efficiency in relation to drought on a semi-arid high-elevation ranch in Wyoming. From 2011 to 2014, we tracked 80 Angus x Gelbvieh cows for calf weaning weight, efficiency (considered as calf weight relative to cow weight), intake requirements, and potential herd sizes relative to 5 stratified cow weight classes (453 kg, 498 kg, 544 kg, 589 kg, and 634 kg). Weaning weights were adjusted to a 210-day and calf sex value. Cow size was a significant factor every year, with different cow sizes having advantages or disadvantages different years relative to weaning weight. Efficiency for the smallest cows (453 kg) was always greater than efficiency for largest cows (634 kg) (P < 0.001) and was greater in the driest year (0.41 � 0.02) than efficiency of the largest cows in the wettest years (0.37 � 0.01). Efficiency change between wet and dry years (?E), was 0.18 for the smallest cow size and 0.02 for the largest cow size. ?E decreased consistently as cow size increased. These results indicate large cows (589 to 634 kg) did not maximize genetic potential in this Wyoming rangeland production environment when conditions are optimum or provide any advantage over small or moderate size cows (453 to 544 kg) across the drought gradient.