Export FBX from Unity - or load from file system

Try to process the textures in Unity to reduce the size, convert to jpg when possible and/or reduce to a smaller size if pngs if large sized or fine detail isn't important.
File->Export->GLTF/GLB to export file ready for upload
Also the free FBX exporter can be used to prefabs - these can result in some large sized results at times as the various materials and textures can be tediuos to match