Increasing index size between documents DELETE operations

I am conducting an index size reduction test according to the documents DELETE operations.
But I got the result that the index size did not decrease, but rather increased.

The test information is as follows.

Query:

‘> db.usertable.find( { } ).limit( 50000000 ).forEach( function( doc )
{db.usertable.deleteMany( { _id: doc[’_id’] } );
})
// half of total documents count

Pre-operation status:

'> db.stats(1024 * 1024 * 1024)
{
“db” : “ycsb”,
“collections” : 2,
“views” : 1,
“objects” : 100000001,
“avgObjSize” : 1167.8795252012048,
“dataSize” : 108.76725771278143,
“storageSize” : 111.40504837036133,
“numExtents” : 0,
“indexes” : 3,
“indexSize” : 4.6610260009765625,
“fsUsedSize” : 118.79688262939453,
“fsTotalSize” : 383.2541809082031,
“ok” : 1
}

During-operation status:

'> db.stats(1024 * 1024 * 1024)
{
“db” : “ycsb”,
“collections” : 2,
“views” : 1,
“objects” : 94936243,
“avgObjSize” : 1167.8795219018725,
“dataSize” : 103.25954676233232,
“storageSize” : 111.40504837036133,
“numExtents” : 0,
“indexes” : 3,
“indexSize” : 8.544868469238281,
“fsUsedSize” : 120.68218612670898,
“fsTotalSize” : 383.2541809082031,
“ok” : 1
}

Index size has increased by approximately 2 times.

Why are these results coming out?