If a 2 ton boat enters a lock which, when full of water but empty of boats, holds 100 tons, then there are 98 tons of water in the lock when it is full. Indisputable.
If a 20 ton boat enters the same lock then there are 80 tons of water in the lock when it is full. Indisputable.
It makes no difference whether you are going up or down to these figures. Going up in the first case requireas 98 tons of water to fill the lock which comes from the upper pound. In the second case the lock reqiires 80 tons of water to fill it.
Going down the figures are the same. The lock requires the same amounts of water from the upper pound
This makes it faster to lock a heavy boat than a light one.