As you upgrade your kernel, the old ones accumulate a bit of space in
To remove them and regain some space, here's a handy command:
sudo apt-get remove --purge $(dpkg -l 'linux-image-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d')
Does the trick all the time, especially for a dedicated but small
/boot partition. It could possibly free up hundreds of MBs.
Always take caution removing old kernels, in case you need to boot with an older one. Double check the packages being removed are actually kernel packages. I've not had a problem with the command above.
For reference, here's some discussion about this command.