The alkaline cell under small to normal loads will last longer
due to its higher capacity, with heavy to very heavy loads
the NiMH Cell will last longer due to less internal resistance
in the cell, Less losses in the cell means higher efficiency
Some NiMH cells are a little bigger. Maybe they are made this way so they can claim they have a higher capacity than their competitors.
I have some C NiMH that I don't use because of their odd shape. The overall length is the same as an alkaline but button on top is shorter and the cell is longer. The button is too short to make contact in at least one of my chargers.
By the way, all my D cell NiMH and NiCad cells are slightly longer than alkalines. They were a very tight fit in my CCrane charger until I filed down the plastic handles in the charger.
eluminator, you may have some flat top C NiMH's. They are used to make battery packs. I have some aero 1/2D's that are flat top. They work ok in flashlight using them as singles, but are a challenge to charge. Most high discharge batteries are flat tops or have tabs. I have to find a good way to charge my 1/2D's and subC's.