Though I haven't done any Rebatching myself (have no interest), it's not much different than cooking.
Yes...every rebatch video I have seen, it comes out fluffy...same with most Hot Process...which is why the bars have a 'rustic' appearance. Which makes sense given that when the liquid heats up, it turns into steam and steam is air. Whisking and beat your soap into submission also produces air. So does stirring, which is why we smack our molds...to release trapped air.
Grating or small chunks doesn't really matter...you could feasible drop in whole bars, it will just take longer. Boiling a whole potato takes longer than boiling large chunks. Boiling large chunks of potatoes takes longer than small chucks. And if you want to thicken a soup...just grate in a potato.
