The CCR model by Charnes, Cooper, and Rhodes (1978)together with the BCC model by Banker, Charnes, and Cooper (1984) are the most popular approaches of measuring eﬃciency among a group of decision making units (DMUs) in data envelopment analysis (DEA). The choice of a DEA model - CCR or BCC - often, if not always, is a difﬁcult decision. To evaluate a DMU’s eﬃciency for both models might be helpful, but it does not always capture the essential issues at stake. In the presentation we propose a comparative analysis of both concepts: How does activity scaling under constant BCC-eﬃciency inﬂuence CCR-eﬃciency. And inversely, how does BCC-eﬃciency behave when activity scaling under constant CCR-eﬃciency is applied. Such ﬁndings of mutual eﬀects improve a DMU’s ability to reassess upsizing and downsizing of activities. We provide optimization models for calculating these adjustments and respective stability ranges and this for both eﬃcient as well as ineﬃcient activities. Finally, scale eﬃciency turns out to be the ideal concept to control the seactivity changes, rather than just CCR- or BCC-eﬃciency. A numerical example illustrates the concept.