NetBSD is a bit easier to run on 8MB systems these days. Still not super easy, but less fat to trim off. I think you might still need to trim down the default kernel, but at least there is a premade config for that (GENERIC_TINY)
If you're daring, it should work on 4 MB machines... ;)
Now, it's entirely possible that even with this minimal config it'll take up too much space, but it may work. I got NetBSD 8 to run on a VAX with 8MB of RAM, although that kernel has many fewer drivers/modules.