The simplest system uses the fewest moving parts to accomplish the goal. Some of Arch's ideas, like PKGBUILDs, are pretty simple. But ultimately Arch is just another typical mainstream Linux distribution with glibc, GNU coreutils, systemd, PAM, etc. These tools are all severely bloated and over-complex, and many simpler alternatives exist which solve the same problems with much less.

As for "small", well, it varies, but Arch systems tend to bloat more with time as the system does not provide much for auditing and cleaning up your system, so the older an arch install gets, the more garbage it accumulates. Arch also tends to turn on as many options as possible for each of the things it packages, so many packages have a lot of optional dependencies made mandatory. This is not a unique problem to Arch; only Gentoo (and maybe Nix and friends) solve this one, and they have many other problems to contend with.

All of this is not to say that Arch is necessarily a poor choice. It's just not simple, nor small.