how does vans have a small-sized json 3d model
This model has one or more textures, and I think it's almost certain that textures account for most of the size difference you are seeing when converting from glTF to three.js custom JSON format. Writing textures into a Data URI (for JSON serialization) in a browser can sometimes create a very unoptimized version of an image.
My suggestion would be to stick with the glTF file — a more efficient format, and three.js itself recommends it — and use glTF optimization tools to bring the size down further or compress the texture.