Just started dumping data from my individual SSDs onto an old laptop one by one so I can reformat the drives as EXT4 instead of xFAT or FAT32 or whatever I had it as before. I wasn't aware that EXT4 was better suited for (and exclusive to) Linux until recently, and I'm still new to it. Is it much more efficient on storage space or something?

Like I said, I've been copying files to and from an old laptop that holds onto them while I format the drives, but this laptop is old af and takes several hours to copy 1TB, so I often leave it going while I sleep. I woke up this morning to no errors or anything, but the files copied to the drive were ~600gb worth when they were ~800gb before. Surely there isn't that much of a difference? Any idea what could be causing this?