I'm not directly measuring loss, since it's pretty small (<0.1 ohm? It has to be quite small for magloops to work). I'm keeping everything in the system (feed and tuning capacitor) the same, and swapping out just the conductive tape. Higher bandwidth implies / is in general a result of more energy loss from the resonant system. Since the geometry is the same and thus the radiation resistance should be comparable, that means we are talking about loss to heating.

This is my understanding, and consistent with physics classes in the past. My understanding could be off, but similar things are noted in various overviews of magloops, e.g. "Broad bandwidth at the resonant frequency is not desirable as it indicates that power is radiated as heat (resistive loss) rather than as radio frequency energy." from (https://www.qsl.net/kp4md/magloophf2.htm)