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The 15" unibody is an inch larger than the old 15"penryn classic macbook pro. I found this noticeable.
But for me, the new 15" unibody is not as portable as it used to be and should be for a 15" laptop. So 17"it is. It's less portable than a 15", but there is no 17" that is more portable than this one.

Also, it is absolutely not the weight. I honestly do not feel a big difference with 1 pound. It's the dimensions if anyone is going to harp on portability.
 
Upgrading from a 12" lenovo tablet to the 15". The thing feels monstrous but I know I will get used to it. 13" MBP wasn't doing it for me since I needed Hi-res screen :p
 
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