I suppose I could have said "you are right" (with the reduced size) but
I was waiting for a new kernel to come along. It has.
240 MB - Total size of /boot
107 MB - Used (with two kernels)
117 MB - Free
This means 48% of /boot is in use.
zypper still said it needed another 10 MB to install the new kernel.
Abort, retry, ignore? [a/r/i] (a):
The adventurous solution would have been to have tried "i" as in "do it
anyway". I copped out, removed the older kernel with Yast -> Software
Management (at which point 50-60 MB was in use) and then installed the
new one using zypper. The 240 / 107 / 117 / 48% figures still hold true.
I'm assuming zypper's calculation of how much it needs is at fault, the
additional kernel should have meant 160 MB was in use. zypper's
guesstimate of 127 MB for a new kernel seems ridiculous, unless there is
some compression going on there during the update process.
There are vmlinux-whatever.gz files in there which take 17-18 MB each,
if one takes 80 MB before compression that would explain a lot.