Imported model files keeps to high memory

Model files, e.g. GLTF files, loaded with some loaders in runtime on a mobile device keeps too much memory. Generally many times of the file size on disk. This becomes a app crashing problem. I am especially doubtful about the texture files that accompany the model file.

Does unity changes the representation of texture image files to a one that keeps more space? Is there a way to represent them as they are? Or this is an inherent thing that an image file will always take many times memory than its size in the memory?

Is there any specific methods to optimize this memory issue?