[Alex Murphy]

See, this is why I don't understand it. In my view, what you would have left is 100% meat. If it's 70% meat, what's the other 30% ?

A "quarter pound" 70/30 patty will shrink during cooking as the fat cooks off. Even if both start off as "quarter pound" patties, the 70/30 patty ends up a much smaller hamburger than the 85/25 patty does. The higher the fat content is before cooking, the smaller the cooked patty will end up.

[NorthMountain]

A "quarter pound" 70/30 patty will shrink during cooking as the fat cooks off.

That cooked off fat is what causes flare-ups when grilling hamburgers over charcoal. 70/30 hamburgers can be downright explosive.