It used to be the Pro (PowerBook) was targeted towards Pro users, and the regular MacBook (iBook) was targeted towards consumers. This also nicely aligned with price segmentation, but that was secondary. The focus was on pro vs consumer. Then Apple added the Air, where the focus, as the name implies, was on thinness.
At some point, however, these categories became less of a distinction between the targeted consumer which happened to align with price segmentation, but instead became all about price segmentation itself. I think that’s where the devices went off the rails. The Pro in MacBook now basically just means this is the most expensive machine in the Apple lineup...
All the lines basically have the same priorities now, with the only difference being the manufacturing budget, which determines what features are available.
But if weight is your primary concern, it makes more sense to go for a small screen. If you go for a larger screen, you're already accepting a bit more weight. How much depends, but personally I'd expect a laptop that sacrifices weight for a larger screen to also sacrifice some weight to have a slightly better keyboard.