The size of the /boot partition is simply copied from debian-installer which chooses 488 MiB by default. You can see the free space on it by running:
df -h /boot
The 124 MB should’ve been the remaining free space on your /boot. Each kernel and initramfs take about 30 MiB each, so with 488 MiB you can install around 8 different kernel versions at the same time.