David,
Well, this 12V Group 31 115AH AGM Fullriver battery (two of which are in our RV) weighs 73.2 lbs:
http://www.fullriverbattery.com/product/batteries/DC115-12And, this 12V Group 31 125AH AGM Lifeline battery weighs 74 lbs:
http://lifelinebatteries.com/products/marine-batteries/gpl-31xt/Which means that the two batteries probably use very close to the same volume of lead.
Which in turn means that the two probably have either -> very close to the same thickness of plates, or if not -> that one of them has enough higher quantity of thin plates (Fullriver?) so as to about equal in weight the other's (Lifeline?) lower quantity of thick plates.
:h