You know, a regular macbook isnt that much bigger.....thicker, yes, but otherwise, not much. Could look into a netbook as well, about the same size, but again, thicker.
Thickness imo is the least important, its length and width that really determinse how portable a device is.
Actually, it is quite a bit bigger in terms of weight (3X). Anyway, we already have three laptops (two 15" MBPs and a 14" Dell), so a 13" MBP makes no sense for me. Not to say it doesn't for other folks trying to decide between it and an iPad.