MongoDB replica set storage difference in primary and Secondary node

Question

Mongodb Replica set(PSA)(commumity edition 6.0 version) my secondary node one database size is 560 GB and same database in primary node 500 GB.. help explain and help out

i will check the db.stats() and indexes number also same

primary db.stats()
 collections: 21,
  views: 0,
  objects: Long("4251179487"),
  avgObjSize: 333.0748902272825,
  dataSize: 1415961140969,
  storageSize: 393951244288,
  indexes: 48,
  indexSize: 178265362432,
  totalSize: 572216606720,
  scaleFactor: 1,
  fsUsedSize: 659525169152,
  fsTotalSize: 965895835648,
  ok: 1,



secondary db.stats()
 collections: 21,
  views: 0,
  objects: Long("4251179487"),
  avgObjSize: 333.0748902272825,
  dataSize: 1415961140969,
  storageSize: 423536803840,
  indexes: 48,
  indexSize: 182481330176,
  totalSize: 606018134016,
  scaleFactor: 1,
  fsUsedSize: 688594571264,
  fsTotalSize: 965895835648,
  ok: 1,