Richard, you are an experienced diver and have been in a wide variety of environments. I would imagine that you are also at your physiologic set point, whatever actually determines that. Have you followed your RMV and seen that it is relatively constant?
At near 6'1", around 265 lbs, non-smoker, fairly sedentary so not in great shape, carrying a compact camera, my RMV on benign tropical dives tends to run around 0.65 cf/min/atmosphere. If I'm really relaxed and low effort, 0.60 is good. If I mildly drift, hang on a mooring line, kneel in the sand or some such, and it's not my 1st dive of the trip, might dip a bit lower.
Some factors are fairly obvious, but their impact isn't always consistent. I can put my hand with fingers extended in a tub of water and thrust it with low resistance. If I ball my fist up and do the same thing, I expect more resistance due to greater cross-sectional area (not stream-lined). We see that in the shapes of dolphins, tuna, etc... The dolphin isn't laterally flattened the way the tuna is, but both are stream-lined to minimize resistance pushing through the water. We're taught to go slow, since the force needed to push through the water rises dramatically with speed.
But there are other large cross-sectional area folks who have better SACs. I know one SB member who's been diving at least off and on about as long as I've been alive, has been a scuba instructor and is a cave diver (so his skill and technique are dialed in to a really advanced degree). I think his cross-sectional area is a bit larger than mine, but I'd bet his SAC is lower than mine.
So I know lower SACs are achievable. How low, the best techniques for getting there and the cost, I don't know.
Per someplace called
BreatheSimple.com:
"Our pulse rate when we’re in a relaxed state is a good indicator of our overall health. A normal resting heart rate for adults ranges from 60 to 100 beats a minute. Generally, a lower heart rate at rest implies more efficient heart function and better cardio-vascular fitness. For example, a well-trained athlete might have a normal resting heart rate closer to 40 beats a minute."
"The normal breathing rate for an adult at rest is 12 to 20 breaths per minute. Exhale times are about 1.5 inhale times. Just like RHR, the lower RBR, generally the fitter you are. Athletes may have an RBR around 8 per minute. RBR is higher in babies and pre-teens and tends to increase with age in adults. Training to promote periods of RBR around 6 minute can help stress reduction."
In theory, this would suggest physical fitness may be a big factor. One of the gym rats will have to get back to me on that...