None of the reefs we've kept over the years have had sumps. That includes a2.5g, 7g, 10g, 28g, 33g, 37g, 44g, 67g, 75g, 180g, and our current 120g. The only tanks that ever had skimmers (HOB) were the 75g and 180g, but that was years ago. We stopped using skimmers because they overflowed from time to time
The key to sumpless success, in our case, is having a low bioload, lots of LR, lots of non-invasive macroalgae in the tank, and a BB so we can siphon out all the crud on a weekly basis.