Comment 3 for bug 1926440

There are only two kernels on the system:
1. 5.4.0-72.80
2. 5.8.0-51.57~20.04.1

And after the kernel installation everything is fine, there is no problem to boot the 5.8.0-51.57~20.04.1.
Processing triggers for linux-image-5.8.0-51-generic (5.8.0-51.57~20.04.1) ...
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.8.0-51-generic
flash-kernel: deferring update (trigger activated)
/etc/kernel/postinst.d/zz-flash-kernel:
flash-kernel: deferring update (trigger activated)
Processing triggers for flash-kernel (3.103ubuntu1~20.04.1) ...
flash-kernel: installing version 5.8.0-51-generic
Generating kernel u-boot image... done.
Taking backup of uImage.
Installing new uImage.
Generating initramfs u-boot image... done.
Taking backup of uInitrd.
Installing new uInitrd.
Generating boot script u-boot image... done.
Taking backup of boot.scr.
Installing new boot.scr.

However install the udev package that triggers update-initramfs and it will cause this failure immediately.
Looks like it's the initrd.img-5.8.0-51-generic.new that's growing.

Maybe the solution is to create a larger /boot partition as you mentioned.