Minecraft wiki entry for Development_versions, associated text: Now output a signal relative to the number of entities on them, always starting at 1 signal strength if any entities are present.
For Light Weighted Pressure Plates, every entity adds 1 strength to the signal.
For Heavy Weighted Pressure Plates, starting at 1, every 10 entities adds 1 strength to the signal.
Example: 11 entities increases signal strength to 2.