It's not one or the other, both are important factors to healthy fish and affect different aspects of the fish's health. Dimensions and footprint are important to ensure that the fish you have has the space to live comfortably and act naturally without being stressed, stressed fish have weaker immune systems, poor coloration, deteriorating health.
Volume is important because the bioload a large fish puts off will quickly pollute a small volume and dirty water will definitely impact your fish's health and growth. Even with great filtration unseen things like nitrates and hormones are only removed by doing regular water changes, and the smaller the volume the more frequent the changes are necessary.
Another important factor to footprint in the fishes stress level is natural cover, rockwork branches, and lighting, etc. If you have a big empty tank with a spotlight on the fish all the time, they will be stressed. If the ight varies and there's places they can hide or provide sight breaks, partial cover etc, they will feel more natural and see more natural behavior.