Comment 4 for bug 1897747

looking further at the space I have available and the compression options, it appears
/etc/initramfs-tools/initramfs.conf
COMPRESS=gzip

update-initramfs -u -k all

may be a better work around in my case, until I can resize the boot partition - using gzip appears to allow me to have 3 kernels installed, which should reduce the need for manual intervention.

https://askubuntu.com/questions/299396/how-to-reduce-the-size-of-initrd-img-on-ubuntu-13-04