Why is the powder room in a $3 million mini-mansion larger than the powder room in my condo? It doesn't have to be any larger in order to perform its function. The space saved by making it smaller could be used for any number of things, such as a larger guest coat closet.

The extra space contributes to a sense of spaciousness and luxury. F seats, even domestic F within the U.S., are already luxuriously wide for the great majority of travelers. (It may be different for "passengers of size," but I've never been one so I'll let them speak for themselves.) A narrow aisle would feel crowded even if it's wide enough for a food/beverage service cart to squeeze through.

The same logic, by the way, applies to white space in print and online media, such as the ten pixels or so around the text of this or any other FT post within its box. It would be more efficient to run text all the way to the edge of the paper or Web page: as a Web designer would put it, {margin:0;}. Why don't we do that? Because a bit of space around things is not, despite what the OP writes in the first paragraph, wasted. Using it to create a sense of not being crowded is more valuable than using it to squeeze in a few more words.

I might lose my engineer's card for saying this, but sometimes it's not all about efficiency.