> Linux emulation
Just to clarify. It's not emulation in the sense it's slower or something. They call it compatibility layer, which is better, but also nobody knows what it means.
This is simplifying a bit, but it's essentially "Linux is just a kernel" so the interface is just Linux syscalls, so the FreeBSD kernel when executing a Linux binary simply answers like Linux (so it has those system calls). How this is used in practice is that on your file system you have Ubuntu/RedHat/... "installed" (so the files and the file hierarchy are lying there) and you either directly or in a FreeBSD jail execute things in there or the binary you have.
I don't know how well it works in the present but in the past that means you could simply download the Unreal Tournament 2004 multiplayer demo or Enemy Territory or other games and just play them as if you were running Linux, 3D acceleration and all, without VM without real emulating, just the kernel providing what a Linux kernel would provide.
Also "heavy" is very very relative and subjective. You can totally have a tiny FreeBSD and a huge OpenBSD and one could argue OpenBSD is "heavy" because it comes with three window managers, an HTTP server, a full blown SMTPD server, ACME client and a ton of stuff that eg a server install of Debian or Ubuntu doesn't come with. But also if you run eg. ZFS things are heavy of course. FreeBSD has however had a time when it tried to strip a lot of stuff from the default install and make stuff either optional or make things available through ports/packages only.
And also there are surprises to be had with such overviews: Eg. your Lenovo laptop likely will give you a more "out of the box" experience on OpenBSD compared to FreeBSD with things like simple wifi setup, sound often doing the right thing (work, come out the right place, etc.) compared to FreeBSD. Also with stuff like HTTPD with ACME being available in a simple way after install I'd say OpenBSD is easier than FreeBSD.
FreeBSD to me feels a bit more like "it can be everything you want it to be". Ports and packages can be complicated if you just start out, compared to OpenBSDs "just use packages" stance. On OpenBSD things in my experience are more of a "it works or doesn't" and when it works often out of the box and/or with docs, while on FreeBSD it's more like it throws some tools into your direction you can build stuff (poudriere, jails, a build system with many options). So it's really cool if you want flexibility but a bit more like you have to figure out if it's possible and how. But that might simply be because of the use cases I used it for.
That said all of them are real general purpose systems, unlike eg. some Linux distributions. So it's not like "OpenBSD is for routers" even though it often seems like it. There are time when the GPU support is better on OpenBSD than FreeBSD's. But also FreeBSD has official NVIDIA drivers, so it's all not that clear cut.