When talking about 4x5 and 645, do photographers say 645 is a cropped version of 4x5? No. Are we cropping the image format? No. To say we're "cropping" when we don't talk about cropping with other formats is inconsistent.
To say that we aren't magnifying doesn't address the fact that the same subject shot with the same lens but in a different format, does appear larger. As photographers, do we want to talk about the size of an object within the image area or some abstract idea about focal lengths?
Another way of talking about this issue is field of view. People are accustomed to the field of view (also called angle of view) of lenses on a 35mm film camera. So when you put a 100mm focal-length lens on a 35mm camera, it gives you a certain field of view. When you put a 100mm lens on a different-format camera it will give you a different field of view.
But the key point worth repeating here is that the focal length still doesn't change; the image within the format does. This is where the 1.5x factor you asked about comes from. The lens on an APS-C-format camera acts as if it has a focal length 1.5 times what you'd expect from the 35mm format.
This applies to any lens. When a lens is designed or "optimized" for a certain sensor, that simply means the lens only covers the area of the sensor and nothing larger. The focal length doesn't change-only the circle of coverage.
All lenses have a certain circle of coverage that affects the format with which they can be used. If you have a 4x5 camera, for example, you must have a lens that can cover the whole 4x5-inch negative. On the other hand, if you're dealing with a small, compact digital camera with a sensor only a quarter-inch across, you can use a small lens with a small coverage, even though the focal length, again, doesn't change.