The difference between mempool "bytes" and "usage"
From the documentation: 

  "bytes": xxxxx,              (numeric) Sum of all virtual transaction sizes as defined in BIP 141. Differs from actual serialized size because witness data is discounted
  "usage": xxxxx,              (numeric) Total memory usage for the mempool


bytes is the size as calculated as vBytes as would be considered for inclusion in a block, usage is the logical size in actual bytes that are consuming memory.