$ ftp https://cdn.openbsd.org/pub/OpenBSD/snapshots/amd64/install75.img
$ doas vnconfig install75.img
vnd0
$ doas mount /dev/vnd0a /mnt
$ doas fw_update -Fv -p /mnt iwx
Get/Verify iwx-firmware-20230629.tgz ...\
/mnt: write failed, file system is full
 failed.

with inlined patch and iwx downloaded I get:

$ df -h /mnt
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/vnd0a     681M    672M    9.3M    99%    /mnt

git blame says that etc/etc.amd64/disktab also was updated on the last
$FSSIZE grow, but i wasn't able to understand/grep what
install360/install.img image is all about

iwx is enabled on arm64 too and its install75.img also doesn't have
enough space for iwx, but I lack the hardware to test the build

diff --git a/distrib/amd64/iso/Makefile b/distrib/amd64/iso/Makefile
index a4cc5dd600e..9968c9b8d30 100644
--- a/distrib/amd64/iso/Makefile
+++ b/distrib/amd64/iso/Makefile
@@ -1,7 +1,8 @@
 #	$OpenBSD: Makefile,v 1.47 2023/12/15 06:03:00 jmatthew Exp $
 
 FS=		install${OSrev}.img
-FSSIZE=		1359872
+# keep free space for large wifi firmware
+FSSIZE=		1396736
 CDROM=		install${OSrev}.iso
 
 MOUNT_POINT=	/mnt