First, the oxygen consumption & body size:
Oxygen consumption has nothing to do with your SAC. In fact, the 5% (volume) consumed oxygen figure is more or less correct only for 1ATA. In depth, you breathe higher PO2, which translates to more molecules of O2 in the same volume - but your body can use only the same amount of O2 as on surface (given the same workload).
You'll get some more O2 dissolved in your blood stream, but the amount is negligible. (once saturated, it stays saturated and then again only the metabolized O2 gets replaced).
The volume of your lungs has a lot to do with SAC, as it pretty much directly affects how much air you'll get through.
That's one of the reasons comparing SACes between any two people (bar identical twins maybe) doesn't make much sense. Someone with 3L VC will have way smaller SAC than someone with 6L VC - assuming all other things being equal.
I have above average lung capacity (6.2 litres VC, when my predicted is 5.something), and my SAC is quite up there - about 21LPM when working in our cold currenty waters. It can go down to about 14 on a relaxed dive, but I plan my dives with 20. I am not exactly a new diver (around 170 dives in last two years).
So, saying someone's SAC sucks (compared to others) is like saying that they suck because they can't bench press 100kgs. It can be easy for someone, and impossible to ever achieve for someone else.
What I'd do in your place is to log my SAC over few tens of dives to watch for improvement, and try to find my Tidal Volume so I'd have some baseline for SAC.
Regards,
Vlad