I remember the Chinook being classed Medium Lift too and then it all changed towards the beginning of the millenium. This is taken from Aviation Week:
In a 2004 report, the Congressional Research Service (CRS) broke down the weight for medium-lift helicopters as those weighing 12,000 - 45,000 pounds, and classified heavy-lift helicopters as those that weigh more. CRS specifically cites the Chinook as a heavy-lift helicopter. Boeing reports early model Chinooks weighing nearly 25,000 pounds empty and 50,000 pounds at maximum gross weight – the highest in the Army inventory.