you should use GLTF files compressed with draco. i use gltf-pipeline to compress assets but there are other tools that go further. you can also compress the hell out of textures, scale them down, etc., which usually are the parts that make it so big.
SketchFab zips the file on the server, sends it over in separate chunks and the unzips on the client-side. They also first load only low-res textures, render, and then send requests for the rest.
(Edit: so basically as @drcmda says - compress everything, ideally only once and only on the server side. If you need HD models on client-side, first send over the low-quality resources, render, then request the high-quality versions in the background.)