1. Case gold is 18k, or 75% by mass; platinum is pt950, or 95% by mass.
2. Cases aren't made to the same mass, they are made to the same volume. Platinum is denser than gold.
This means there is approximately 40% more mass of platinum in a pt950 case than gold in a 18k gold case for the same watch.
Of course, this doesn't explain the more expensive price when the cost basis is less than half for platinum, but it makes the gap closer...