Modern operating systems are not just a kernel that can run programs. They ship with a wide array of programs and a large desktop environment. And code size is probably a relatively small part of the equation too. All these built-in programs and things come with art assets. Sounds and graphics that can scale to 8K displays while still looking sharp. Some of that is vector drawings that will scale indefinitely, but there are also PNGs and bitmaps in there.
A truck is not bloated because it is bigger than a motorcycle.