Robocopy Odd behaviour

I had to replace a drive and once done, I made a backup or the entire drive using robocopy with this command:
robocopy "D:" "L:\Drive_D" /R:10 /W:30 /NS /NC /NDL /E /MOT:5 /MT:8 /XD "D:\$RECYCLE.BIN" "D:\System Volume Information"
Took a while, the destination drive was an empty portable 2TB WD Elements drive and I mistakenly used a crappy USB cable and there are many small Visual Studio files, when it finished I got this as the result (wishing I knew how to align columns, so here's an image)

I then used Windows to get the stats on the drive and the destination folder

D: (source) 204560 files. 30863 folders, 167 GB 180,242,440,229L:\Drive_D (destn, was blank and folder was created just before backup) 204560 files, 30860 folders, 210 GB 225,870,392,297

Doesn't make sense to me why the backup copy is significantly larger. Any ideas ?
Win10 x64 build1909 8GB ram Q9550

What are the sector sizes on each drive that will effect it

Samuria said:

What are the sector sizes on each drive that will effect it

From MSInfo32:

SourceModel WDC WD40EZRZ-19GXCB0 ATA DeviceBytes/Sector 512
DestinationModel WD Elements 25A2 USB DeviceBytes/Sector 512

Check source and destination against each other.

Set up a "Synchronise" backup in SyncBackFree and just do the first "Run". This will compare the drives against each other and give a list of which files have to be copied to/removed from each of the drives for them to match up.

Don't do the final "Continue Run" unless you want the drives to match each other exactly.