Companies increasingly collaborate with social-media influencers to promote their products and services. Compared with micro-influencers, however, macro-influencers face difficulties maintaining authenticity and engagement. Given the changing regulations concerning these partnerships, the objective of this research is to investigate the moderating role of sponsorship disclosure in understanding large communities’ perceptions of authenticity and engagement. Using real data, Study 1 confirms the impact of community size on engagement in an inverted U-shaped manner and highlights the moderating role of disclosure messages. Study 2 determines perceived community sizes of micro and macro communities, and uses an experiment (n = 1,004) to highlight the mediating role of authenticity. Disclosure messages improve engagement for macro-influencers only when the message is published by the influencer (versus the platform or no message).