Well, I did it. And I'm wrong, it's not just a 5% reduction in capacity.
I placed a sandwich bag into the mono mini pochette and filled it with water to the point where I can push the 2 top edges together without water getting pushed out past the zipper line. I wasn't going to try to zip it up and risk the plastic getting caught on the zipper so there will be some error.
438 g of water can fit in the mono mini pochette (or roughly 438 cc or mL - just going to assume the baggie weight is negligible)
View attachment 4701705
348 g of water can fit in the pastel mini pochette (or roughly 348 cc or mL)
View attachment 4701706
That's nearly
20.6% reduction in capacity from the mono to the pastel.
I took out the baggie with water from the pastel mini pochette and placed it into the mono one. You can see the water line doesn't even come close to zipper!
View attachment 4701707
Just for comparison, based on the website dimensions of 15.5 cm x 10.5 cm x 4.0 cm, the capacity based on exterior dimensions should be 651 cc or mL. but it's not a perfect rectangular prism so you get some reduction when we're talking actual capacity.