The new MacBook and MacBook Pro may be moulded by hand from a single piece of adamantium or whatever, but how do they size up to their previous versions? The new MacBook Pro actually has a larger footprint (X Y) than the old MacBook Pro, but it's also slightly thinner (tough to tell in person). But the new MacBook, on the other hand, has a nearly identical footprint (X Y), but seems much thinner than the old MacBook. And the numbers agree.
In other words, to the naked eye, big improvement on the MacBook dimensions. Minor improvement on the MacBook Pro dimensions.
3.3mm doesn't sound like much, but combined with beveling, it makes the new MacBook look and feel much more like premium device.