How to verify size of pigz (parallel gzip) archive contents?

I created some pigz (parallel gzip) - home page - compressed archives of my SSD disk drives. (compiled version 2.8) I called one of them **4TB-SATA-disk--Windows10--2024-Jan-21.img.gz** which says the size of the drive and the OS installed on it. Alternatively, the size is of course smaller in TiB which may be comfortably shown by fdisk and tools like it (Disk /dev/sda: 3.64 TiB). I knew from the past, that listing the compressed file cannot show me the real size of contents. It will show 2.2GB or similar nonsense, even in Archive Manager for GNOME. It likely has something to do with gzip structure limitations. However I had some doubts about the real size of the contents, therefore my question states how may I verify it?