I have a 2006 Frontier that I use to tow with. The 2006 is identical mechanically to the newest Frontier. My 2006 had a stated frontal area limit of 60 sq ft. The newest, as the OP states is 30 sq ft. Why the change, Nissan ?
I suspect it has to do with meeting J2807, which came out after my truck was built.
I don't worry about it. I pull either my 7' wide 10' tall Funfinder, or my similar size horse trailer with the truck. I have 155K miles on it, about 60K of that towing. The scangage on the dash shows real-time water temp, and it only runs 3 to 5 degrees hotter when towing vs empty. Good to go with either trailer.
I don't know the exact answer on how they determine frontal area. As others have stated, there are several factors such as the slope or other shape of the front of trailer. On my camper, there is no part of the front that is straight up and down flat. On my horse trailer, there is. I don't over think it. It either works or it doesn't. In my cases, it works. Good enough for me.
BTW as a matter of reference, our 2006 F350 diesel dually has a frontal "limit" of 60 sq ft.