Roshi and Piccolo moon busting is only weird when you look at the series in a vacuum.
The contextual evidence off The universe is that ki can blow up big stuff even when the characters are weak. It was established early and often. Season 1 of DB had mountain busting and it ramps up from there to the point where Nappa, a guy massively above anything in DB, with a gesture is flattening cities, and shaking continents with Vegeta being unharmed right next to him. That blast was well in excess of Tzar Bomba.
Hell, let's toss out moon busting completely. Lets ignore it. Piccolo and Roshi were island busting, city busting and mountain busting in DB. In Z, the Light Grenade did way less damage than that in terms of collateral. Way less than the first Special Beam in fact despite being a charged supposedly non compressed blast from a guy thousands of times stronger than Nappa.
This is one example of many, but do you feel it was less powerful than Raditz dwarfing mountain ranges with his blasts?
Edit: do you have any explanation that makes sense for attacks like that beyond "Toriyama is kinda dumb and lost sight"?