i wasn't trying to say you're wrong, just that considering how many other off-the-shelf parts they used to get the model S built quicker (ie: mercedes switches on the column), it would make sense to use a battery that's readily available. i figured they would continue to do some of the same for the model 3 to drive its cost down, and to make it easier to service.
that said, i had to search a bit, but i finally found a chart that lists the actual Ah rating of a normal car battery:
http://s7d5.scene7.com/is/content/CanadianTire/BatteriesSpecSheet_Automotive15314_ENpdf
according to that, a typical group 51 battery can range from 35-45 Ah capacity. physically, the group 51 is indeed 30-50% smaller than a "typical" car battery. conveniently, that chart also lists physical dimensions in inches. FYI - the only difference between a 51 and a 51R is that the polarity is swapped so that (-) is on the left for one of them, and it's on the right for the other. the group 24 i mentioned is rated around 70Ah, which just happens to align almost perfectly with your stated 30-50% size difference.
also on that list is the mazda miata, which had its own custom battery from 1989-2006 (it vented externally since it was mounted in the trunk) and is even smaller than the group 51, and has a measly 26Ah rating, yet people run low current stuff from those while parked as well. i didn't have much more than just a stereo and clock in my 97 miata, but the only time i ever had a dead battery was when i left my lights on by mistake. and then i had to learn how to push-start a car. that was real fun...
another thing is that for things to work while the car is parked (ie: remote door locks) it has to get power from somewhere, and that's typically the 12v system. no, i haven't seen a wiring diagram for a tesla yet (has anyone who doesn't work at tesla?), but it wouldn't make sense to keep the inverters powered up and use up range from the traction battery just to listen for door lock signals or to store settings in nvram (radio presets, wifi settings, whatever). obviously i could be way wrong, but then why would they even need the 12v battery at all? they could just use a simple step down transformer and sip from the traction battery to power any 12v systems (12v accessory sockets, radio, wiper motor, whatever). but based on what i've seen in toyota hybrids, the 12v system is pretty much completely independant of the 300v hybrid system - it just uses the inverter to step down to 12v to run those systems when the car is on, since there's no alternator, and you'll still need those 12v items while the engine is stopped.
no, it's not always easy to run wires through the firewall, but if the manufacturer did it, then so can anyone else. just depends on how much you're willing to take stuff apart, or to just poke holes in existing wiring grommets to sneak your wires alongside factory stuff. when i was a mechanic, i installed a LOT of alarms, stereo systems, DVDs (headrest and overhead), exterior lighting, etc... i've had to drill through plenty of metal to run wires, so it doesn't scare me anymore.
i'm not trying to argue. just saying that nothing is impossible. it might not be EASY - definitely can't argue that, but impossible is a strong word.
