What do you gain with picking a 12-inch display over a 13.3-inch display?
It really comes down to two things
1. Weight
2. Tolerable Performance
The Macbook I have has a 1.4 GHz Core i7 and 16 GB of ram. Very rarely do I feel that I have issues with the daily things I need to do with the mac, which are some light programming/scripting/SQL and even hums when I have to do spreadsheets and manage multiple websites.
Given the performance is very good by real life standards, the weight is really the big issue to consider.
This is a considerable amount of difference between 2.03lbs and 2.75lbs. That's 35% lighter, that's a huge percentage when you have to drag your laptop around everywhere with you. I would gladly pay up to $1000 for that weight cut because of how much better my quality of life is with a smaller and more portable Macbook.
For me, I need to have my computer with me everywhere I go: dinner, hiking, vacations, etc. Having it do everything I want in a reasonable performance threshold and being 35% lighter, the Macbook is still by far the superior product.
It also appears anecdotally that the smaller screen is more popular in Asia than in US/EU (for both Mac and Windows).
This is bizarre since the 2 machines are not immensely different in sizing, but it's something about the feel. Obviously this is highly subjective but this is making me consider a 12" MacBook to have as my personal, non-workhorse machine.
The smaller laptop is lighter, and when we have airlines aggressively enforcing 4kg limits on carry-on, every gram matters. I put my chargers in checked baggage, while my camera, lenses and laptop live in carry-on.
An extra 200g of laptop means leaving behind part of my camera. Not price sensitive about one-off purchase, but I am price sensitive about more frequent purchases!