The ISO includes the kernel and initramfs twice. Once in ISO 9660, and again in FAT (`efiboot.img`). This is because systemd-boot doesn't have file system drivers, so it cannot load the kernel and initramfs from the ISO 9660 volume. Since the releng initramfs image is huge (~45 MiB), the ISO is wasting a lot of space. Possible solutions are: - Replace systemd-boot with a boot loader that has a driver for ISO 9660. - Find some way to reduce the initramfs size. - ~~Wait for [systemd issue 15617](https://github.com/systemd/systemd/issues/15617) to get implemented, set the ISO 9660 volume as an [Extended Boot Loader Partition](https://systemd.io/BOOT_LOADER_SPECIFICATION/) and place an [ISO 9660 EFI driver](https://efi.akeo.ie/) in `efiboot.img`.~~ After !160, `-iso_mbr_part_type` can't set the ISO partition type anymore.