Then why not get the 15"?! I honestly don't understand why the 13-14" screen size exists. If you want something really portable it's got to be 12" or less. If you want something you can do serious work on too it must be 15" or more...

(The only thing more nonsensical than this is the wide screen AR when we know a 4:3 ratio would always be superior. Yeah, power users will customize their UI to get rid of the crap wasting vertical screen space... but those very same power users will also want to see long batches of code or spreadsheet cells, or text, so they'd still crave for more vertical space.)

And 12" is just a little too small for my preference, though not wholly unbearable.

My 15" and larger machines have been much harder to travel with easily. For productivity, I have usually preferred the 15"+ machines. If that were the only consideration, then I wouldn't think twice. But they have been a pain to haul.

I have a 14" T470P (2560x1440) and I do 'serious' work on it all the time, its more than adequate for when I want to work away from a desktop.

What about something semi-serious... like Photoshop or Illustrator which can take reasonable amount of resources, but far fewer than a video editor or an IDE. A 13" with a really good screen can handle this task well and doesn't need the added heft of a 15".

I'd buy that even if it adds one centimeter to the height (gasp ! Now I can have an RJ-45 port and more than one USB port ?!).

yes there is. area and volume inside the chassis increase a lot faster than the length of the diagonal. the limiting factors for performance are heat dissipation and power. a bigger chassis can evacuate heat faster and house a larger battery that can sustain high power use for longer.