The capacity of physical activity (PA) measures to detect changes in PA within interventions is crucial. This is the first study to examine the responsiveness of activPAL3™ and the International Physical Activity Questionnaire (IPAQ; Short Form) in detecting PA change during a 12-week group-based, men-only weight management program—Football Fans in Training (FFIT). Participants wore an activPAL3™ and completed the IPAQ pre- and post-program (n = 30). Relationships between change scores were assessed by Spearman’s correlations. Mean or median changes in PA were measured using paired samples t-tests and Wilcoxon signed-rank tests. Responsiveness to change was assessed utilizing Standardized Response Mean (SRM). Both device-based and self-report measures demonstrated significant changes pre-post intervention, although these changes were not significantly correlated. The SRM values for changes in activPAL3™ metrics were: 0.54 (for MET-mins/day); 0.53 (for step counts/day); and 0.44 (for MVPA/day), indicating a small to medium responsiveness to change. SRM values for changes in IPAQ scores were: 0.59 (for total PA mins/day); 0.54 (for total MET-mins/day); 0.59 (for walking MET-mins/day); 0.38 (for vigorous MET-mins/day); and 0.38 (for moderate MET-mins/day), revealing a small to medium responsiveness to change. These findings reveal that two commonly used device-based and self-report measures demonstrated responsiveness to changes in PA. While inclusion of both device-based and self-report measures is desirable within interventions it is not always feasible. The results from this study support that self-reported measures can detect PA change within behavioral interventions, although may have a tendency to overestimate changes compared with device-based measures on absolute values, but not standardized response values.