Reading from stdin `dd` copies more data than reading the same thing from file

Question

I encountered weird behaviour of the dd tool:

1. I downloaded compressed "newsystem.iso.xz".
2. I ran xz -d --stdout newsystem.iso.xz | dd of=/dev/sdb bs=1M conv=sync.
3. The decompressed "*.iso" file weights about 4.5GB. I expect this to fit on the usb. Instead dd fails with "Not enough space" error, saying that it wrote 8.1GB (the usb is about 8GB)

But then I downloaded the already decompressed "*.iso" and ran:

1. dd if=newsystem.iso of=/dev/sdb bs=1M conv=sync
2. Command succeeds after writing the expected 4.5GB.

I can verify that xz -d --stdout newsystem.iso.xz | shash512sum is the same as shash512sum newsystem.iso.

The sync option means

sync pad every input block with NULs to ibs-size

The dd version is "GNU coreutils 9.0" and the iso image is "FreeBSD-13.0-RELEASE-amd64-dvd1.iso".

Thanks in advance!