You're right, but there are so many people out there who talk about how much smaller their 12" PB was, and how they refuse to get a MB because it's too big.
I have no idea what these people are talking about. I'm not a particularly strong person, but I can't tell the difference between the 12" PB and MB, and I've owned them both. I'm sure the difference exists, but surely the 1 cm difference in length and width, and the very slight weight increase isn't affecting portability.
And when you start talking about the weight of a product, think of it this way: All these laptops use the same components. We're talking about mobo, cpu, HD, fan, keyboard, LCD and the external casing. Are Sony using special components that weigh less? Probably not.
I don't want to buy a casing that weighs less. If they're all using plastic, but Apple is using more, then that's fine with me. I wouldn't want something less sturdy. But if everyone else was using carbon fibre or something while Apple was still using this same plastic, then sure, I guess you really should be asking Apple some questions.

Carbon fibre wouldn't provide much more rigidity anyway since we're not using this plastic edge-on, but whatever.