Researchers orbit a muon around an atom, confirm physics is broken

Related discussion on the same study: https://news.ycombinator.com/item?id=12270483

For protons, the muon-derived radius is 4% smaller than for the electron-derived radius. But for deuterium, the discrepancy is only 0.8%.

Is there some obvious scaling argument why adding a neutron should reduce the anomalous effect by a factor of 5? Actually, why is it that the deuteron's charge radius is so more than 2x the proton's? It obviously doesn't work like you'd expect something volumetric to work.