Why is a bitcoin node pruned to 550MB taking up 10GB+?
$ du -smc blocks chainstate index | sort -g
A pruned node deletes historical block chain data it already validated. However the blocks are not the only data stored by a node. In order to be able to validate blocks a node needs to maintain a UTXO set (here in the
chainstate folder). This data cannot be pruned.
Nowadays the most part of the additional disk space usage comes from the UTxO set. But as @Murch points out in the comment a node will always keep at least 288 blocks, which can also technically exceed 550MiB.