Why do two different fully synced Bitcoin Core nodes differ in the blockchain size?
The size of the blockchain on disk can vary depending on how many stale blocks your node has seen. Stale blocks are not part of the main chain, but they are still stored on disk by your node. A new node will have fewer stale blocks than an old, long running, node. Well connected nodes are more likely to see stale blocks than less well connected nodes. So it is possible that your node simply has not seen as many stale blocks as other people's nodes.