When placing snow layers, the hitbox changes to match how many layers it has, but no matter how many you place, you get the same amount back if you use silk touch. When you place 8 layers down, the hitbox changes into a block, but when you use silk touch on it you get a snow block and not 8 snow layers. (if you open f3/debug menu it shows the 8 snow layers as snow layers and not a snow block)