I've been measuring the kernel sizes of FreeBSD, Linux, NetBSD and OpenBSD and found surprising but also interesting results. I took the latest stable versions of each into account, by the way. Below is a list of them (from bigger to smaller).
* Linux 5.15.x = 1.1 GiB * FreeBSD 13.0-RELEASE = 502.2 MiB * NetBSD 9.2 = 355.1 MiB * OpenBSD 7.0 = 297.8 MiB
(Everything else is an excercise left to the reader)