Part of it is just dose. Moderna had different Phase 3 trials than Pfizer and didnt hone in on dose as much, so its shot was probably overkill. But seeing how it now seems to have more longevity, maybe that overkill comes with a bonus staying power.

Im not sure it quite works that way but maybe it really is as simple as "less more often" = fewer side effects, "more less often" = more side effect, less often"