That’s not a fair comparison at all. Busybox is specifically optimized for size, and to accomplish that, leaves out a large number of GNU compatibility features

Such as? busybox provides a nice interactive shell, awk, bc, wget and much more. I know GNU awk has a lot more features than posix awk but awk is not part of the uutils anyways.

busybox also implements [[ from bash, none of this is provided by uutils or coreutils.

EDIT: busybox also provides grep while the uutils/coreutils don't.

I’ve built it that way now and that puts it under 7 MiB; still much larger than busybox, but it shows how much the optimization choices matter.

I'm assuming this uses -Os which means performance hit, (iirc busybox also uses -Os so it is fair comparison), still we are looking at 7x larger binary.