I'm no particle expert, but I am a little skeptical about the interpretation of these experiments. They are interested in the spatial spread of charge in a pro

I'm no particle expert, but I am a little skeptical about the interpretation of these experiments. They are interested in the spatial spread of charge in a proton (or deuteron). And they call this the "size" of the proton. Good. And they reason the more spread out it is, the smaller its effective charge in the electromagnetic interacton between it and some bound particle (electron, muon). Good. So they reason that spectroscopy on that particle can tell them about that spatial charge spread. But that's assumes that the only force involved is electromagnetic. Any additional short-range interaction between the two particles can shift that specturm. And we expect those shifts to be larger for the muon just because it overlaps more with the proton even if (a) the proton's size is unchanged and (b) the interaction strength with the elecron (at a given distance) is just as strong. Now if there is no extra ineraction, then their methods do measure the proton size. But then they have to explain why it changes -- which would require an extra interaction.