youswer because windows wants 100Mb, Solus may keep up to 4 kernels, some users also install other OS, etc. (everything is installed on the first ESP partition by default and the Solus installer currently won't let you select another partition, actually even today most installers aren't very flexible about this)
Not all kernels ships the same modules nor are compressed the same way.
If size matters for you then look for solutions that focus on that specific matter like damn small linux for example.
System requirements aren't defined just for the fun of making documentation!