Thanks, great work! I thought the second gen was the same thickness for some reason, but it looks like you're right, they increased by 1 mm to accommodate the coaxial. I also found the third party number for the third gen didn't match Omega's website, they list the thickness themselves for the ceramic models.
So the updated thickness numbers and percentage gain over first gen:
First Gen SMP: 11.5 mm
Second Gen SMP: 12.5 mm (+8.7%)
Third Gen SMPc: 13.1 mm (+13.9% or 4.8% over second gen)
Fourth Gen SMD: 13.6 mm (+18.3% or 3.8% over third gen)
I assume the second gen thickness increase was entirely due to the co-axial escapement, and the fourth gen's increase is probably due to the sapphire caseback and in house movement. And it actually had the smallest increase of any generation!
But what was the third gen's excuse, I wonder...
Anyway, I still hope the next gen can shrink down to under 13 mm with a date function.