Depending on size and quantity of the textures in your model, the Sketchfab viewer may load the full resolution versions or smaller versions that we generate when you upload the model or add textures in 3D Settings.
It is not possible to force the viewer to load the full resolution textures by default. It is automatically determined based on the balance between visual quality and performance.
This is decided based on a "pixel budget". If we detect that you are viewing a model on a mobile device, the pixel budget is 6144x6144 pixels (the equivalent of nine 2k textures, for example); otherwise, on desktop computers, the pixel budget is 12288x12288 pixels. This means that if all the textures "fit" into a single canvas of that size, the model will load in "HD" mode (full resolution texture). If the textures do not fit into that size, the model will load in "SD" mode (smaller resolution textures).
