You’re right, downscaling textures is a part of reducing the scene size when the scene is uploaded to the web. Textures are downscaled based on how stretched they are in the 3D space. To force using maximum allowed size (2k x 2k) for a particular texture, place a small square, say 10cm x 10cm, in some hidden area of the scene, and apply the texture so that roughly one pixel of the texture occupies the whole square. Then, re-upload or re-bundle your scene.