Weight matters. Footprint matters. What you mean by "portable" matters most of all. If all you're doing is walking from you back door to the car and then from your car to the garage elevator, you could almost carry a desktop around.
If "portable" means carrying the computer for a couple of miles on foot and then on public transit, 15" vs. 13" can make a big difference both in terms of weight and in terms of what it will fit into with a reasonable degree of protection.