Abstract Introduction Changes in muscle size are often measured as a change in muscle thickness (MT) or as a change in muscle cross‐sectional area (CSA). Differences in how muscles are measured is an important consideration as different absolute and relative changes in muscle size may carry different meaning. Purpose The purpose of the present study was to compare acute changes in MT and CSA in the rectus femoris following five sets of knee extension exercise. Methods Thirty‐four participants visited the laboratory for one visit. Participant's dominant leg was placed into the exercise condition, and their contralateral leg was used as a non‐exercise control. Ultrasound images were taken in both legs, before performing five sets of unilateral knee extension exercise in the dominant leg. Following exercise, participants had ultrasound images re‐assessed. Pre and post values of MT and CSA were calculated from all images. Results Muscle size increased in the exercise condition when measured as MT (mean change [95% CI] = 0.404 [−0.347 to 0.460] cm, p < 0.00) or CSA (mean change [95% CI] = 0.734 [0.522−0.946] cm 2 , p < 0.001). In relative teams, MT increased by 25.5%, and the effect size of the change was 2.49. For CSA, the relative change was 37.3% and the effect size of the change was 1.20. Conclusions The relative change in MT following five sets of knee extension exercise was smaller than the relative change in CSA; however, the effect was larger, suggesting that the smaller relative change may have been easier to detect.