Maize and sorghum are major crops in dry land farming systems where grain yield is often limited by water availability around anthesis. Sorghum is generally perceived to be more drought tolerant than maize, but it is unclear if that is associated with a difference in amount of water use (transpiration, T) due to the shorter stature of sorghum, or with a difference in biomass production per unit water used (transpiration efficiency, TE). The objective of this study was to determine if sorghum and maize differ in TE, and if so, whether any difference was associated with plant stature or with components of TE (conductance or photosynthetic capacity). Individual plants of eight maize hybrids, seven tall double dwarf (2d) sorghum genotypes, and 14 short triple dwarf (3d) sorghum genotypes were grown in lysimeters at Gatton in 2011. Plants were well watered and harvested 5±1 days after flowering. The number of fully expanded leaves on each axis of each plant was recorded twice a week, and the size of each fully expanded leaf measured. At harvest, total T, and shoot and root dry mass were measured for each plant. Photosynthetic capacity was calculated as biomass produced per unit leaf area (B/LA) and conductance as T per unit leaf area (T/LA). Sorghum had a significantly greater TE than maize, but this was not due to the greater plant height of maize, because 2d and 3d sorghum had similar TE. Genotypic differences in TE were observed for sorghum, but not for maize. These differences in sorghum were associated with differences in B/LA, rather than T/LA and were not obviously linked to the drought adaptation trait stay-green. As stay-green expression can be a consequence of plant size, this indicates that TE and plant size are potentially independent mechanisms of drought adaptation.