A little matter turns into a LOT of energy, so imagine those planetary pieces turned into energy.
Also, on the subject of Adonalsium, we've seen that Shards have a moderate physical, tiny cognitive, and massive spiritual aspect. We can assume the physical simply comes from the body of whoever holds the shard, but the conginitve is present on its own. So would all 16 cognitives put together from Adonalsium's mind? That would remove Adonalsium's need to be a human, and it would also mean that those idividual pieces could be messing with the current holders minds beyond simple intents.
Alternatively, Adonalsium had a physical "element" (Reference, the Letter) but lost it upon shattering. To reform it would probably need a physical aspect unlike anything ever seen before, possibly made of something totally foreign to the world, making it virtually impossible to reform. Probably for a reason.