I had a similar experience when I recently switched from my leisurely, buoyant, SP twinjets to a pair of Apollo biofin XT pros, and actually packed the latter monsters all the way to the Galapagos and back (both for their coil-spring don-and-doff ease on --and hanging onto--small dive boats, and their stiff power in current). I wondered why I was able to dive with less weight than anticipated in that quite saline and cool water.
Recently, I found out why. I wore these fins to one of our Saskatoon Triton Dive Club pool sessions, but forgot to take along the belt and three or four pounds of weight I usually need to stay under for breath-hold UW hockey and such, but had no problem staying down. The usual buoyancy check--standing vertically in the water and slowly exhaling--demonstrated the reason: with the new fins I barely floated at eye-level with a nearly empty chest, the same as I would have wearing 3-4 pounds with the old ones.
While this wasn't a completely controlled and scientific test because my weight and body make up as well as the exact temperature and make-up of the water may have changed in the months between wearing the different fins, it does correct for displacement and the simplicity of the basic physics involved tends to indicate that your guess may be right. Why not try it!
P.S. Now I understand why kicking with these fins when also wearing the usual ankle weights in drysuit dives is tiring. You may, indeed, want to dispense with ankle wights when wearing heavy fins.
