Right... because no one ever has two rocks in their scene... heh. Two rocks per sheet, nearly 100% of the pixels being used, if you really wanted to use 100% you actually could using the baseball unwrap. I wouldn't use a rectangle texture, its a waste. Most games are set up to expect a specific size. So a material slot…
I had similar issue with TextMesh pro. When I made a 4K texture with the font it looked blurry when scaled down. So I made a 512 texture for smaller stuff.