Why is there a tip saying “file is too large” when we save a file into FAT32 partition which has enough space? I use the software -MiniTool Partition Wizard.

How big is the file? FAT partitions have a limit on the size of a file that can be saved. For FAT32 that is 4GB (gigabytes), so most DVD images cannot be saved on a FAT partition for example.

I've crimped a 7z to the 4GB limit and it wouldn't even store that on the drive...
(the file was EXACTLY 4096.0MB)

you're better off sticking with 2GB-sized files on a FAT32 formatted drive