My 12" MacBook actually fits under the seat on a flight standing vertically. This makes sense since the 12" MacBook is 7.75 inches deep, but there is 8 inches of spaces below an airplane seat. It also fits perfectly in my travel pouch which is the size of smallish purse. Ironically, the MacBook Air does not fit vertically under an airplane seat as it 8.5" deep. Some may say it's kind of a little thing in the greater scheme of things, but nonetheless it's a demonstration of the 12" MacBook's increased portability vs. existing models, which makes it a pleasure to take for traveling.
Mind you, these days for many trips, as mentioned I often just take my 11" iPad Pro.