Way back, I assumed it was a lot higher because I just didn't like the idea of Nail being a better assimilation than Kami. Maybe because merging with Kami was supposed to be such a big deal to Piccolo.
But when I read Krillin's quote on how Piccolo's huge improvements "would make the assimilation that much better", I just saw no reason to make it higher than 2x. Krillin's quote is pretty simple, but that alone just told me the boost didn't have to be as ridiculous as I once thought it was. So with that, Piccolo's increases throughout the yrs seemed to more of a reason for the huge spike in power than the assimilation alone.