Why is temporary database reported smaller than actual size of the disk ?

I have notified that on several servers the IRISTEMP database is reported as only a few GB in size while on the disk where it's located, the IRIS.DAT file is much bigger (eg: 3GB reported in Portal (including free space) while file on the disk file is 121GB). The last modification date of IRIS.DAT is recent so I'm not looking into a location no more in use.

Is there an explanation for that difference in size ? 

I know temporary databases are special in a way they are not always stored on the disk, here is what InterSystems says about it :

Changes to temporary globals are not written to disk. Instead the changes are maintained within the in-memory buffer pool. A large temporary global may be written to disk if there is not sufficient space for it within the buffer pool.

Still it does not explain the size difference between portal and physical disk.

Also : I saw that this database get expanded multiple times, sometimes 20 times in 2 seconds (eg : 30, 33, 37 ... until 1000MB). Would it make sense to change expansion size to something else than default ? (eg: 300MB).