The 3D models are compressed with 'Draco' compression. So for example the largest asset is a model of the entire world which is 333 kB. It consists of 81k vertices. Unoptimized that would mean at least 3 32 bit floats per vertex, which would be at least 972 kB uncompressed. So Draco compression is doing a pretty good job getting the model sizes down.
I'd have to dig into it more but I suspect if they wanted to they could have trimmed the size down quite a bit, but it may not be worth the effort. They could have used more repeat 3D models, but instead it looks like they went for a more artist driven look where most of the world is uniquely modeled.
It also looks like they're sending a bunch of lower res levels of detail per model over the network. That also is a tradeoff. Still with network speeds as they are nowadays it may be faster / simpler to send those over the wire each time instead of regenerating the level of detail.
The textures are a similar story. Well compressed but they're sending procedural noise over the network. Those could be generated but it's also trivial to send them over the network.
https://almanac.httparchive.org/en/2024/page-weight#fig-37
So, 5-15 for a video game (vs a static document) is pretty good!
I'm not trying to be a demoscene smart-ass here, either (I expect demoscene masters would've packed the equivalent in under 1MB). Just that, models are low-poly, the world is small and made up from pieces that feel simple to describe; as long as you're not trying to bake everything into a static set of meshes, but willing to encode them at a higher level, 5MB seems like plenty.
For a dimensionally reduced analogy, the 2D equivalent would be a perfect example of an image that's very large in raster form, but quite small in vector form.
Are my intuitions widely off here?
Note that this does not make it any less impressive - on the contrary, I'm amazed by how much detail and soul is there to this world, despite apparent simplicity. I'm also amazed at how navigable this world is. I've made many stupid moves that I was dead sure will wedge me between walls, or get me stuck in a nook with no way of going back; but none of that happened. They must've put a lot of thought into the design, and it wouldn't surprise me if they manually mapped out the world to ensure there's no one-way paths.