the carts I received weigh around 212g +-
Well its pretty obvious there isn't going to be 200ml of ink in the cart if it only weighs 212g. It would mean the empty cart weighs 12 grams!
Considering that my 300ml cart when empty weighs 74g, and the other poster mentioned his PFI-306 is 114 grams, which is designed for 330ml, then the PFI-706 cart must be at least another 20 or 30 grams over this to account for the extra length the cart needs to be. If we use a very round figure of 150g, you are clearly getting less than 100ml in each cart, give or take.
To be fair to the guy, if he thinks it was 25% full, and is basing this on just the little picture icon, then this probably easily has a 20% margin of error. There are only about 5 or 6 different levels of full on that graphic icon, so anything above the lowest might look like its at least 25% full.