I believe the default used to be 500MB~ and MS changed it to 100MB as of 1709. If you do the simple install version I believe Windows decides where to put the partitions. It may have detected the GPT partition and decided to use it. I don't think the logic for the installer is really documented. When in doubt when installing Windows it's best to physically unplug all the drive except the one you wish to install on. Typically it's more convenient to have all the boot loaders on the same partition, so you don't have to select the disk to boot from each time you wish to switch OSes.