So a car charger is doing a step-down of 12V DC current so its inevitably going to be more efficient, it is basically just a resistor which discards the excess voltage (e.g. dual port input = 12V, output = 2x 5V, discard 2V). You can run say 2 lots of 2 ports in parallel to make parallel lot of 2 x 5V who each share the current, etc. So a car charger has its "easy" which = smaller.
A mains charger from AC 110V-250V has to run an inverter, it is stepping down the voltage to step up the current so its huge Volts and tiny Amps becomes small Volts and larger Amps, so that inverter creates heat, then it must flatten out the AC to DC, smooth it out which involves (guessing) a little capacity which stores/releases energy also causing some heat.
Correct, more efficient inverters create less heat so you can make them smaller.
Also, "compact" and thermally efficient are at odds, to get rid of heat the least-square the better, you'd want a larger surface area to spread out so say more of a thin charger (but other 2 dimension large) is more thermally efficient.
The Aukey I mentioned has it easier as its only 12W, that one does fine for, say, just a phone, or just a phone and a BT headset.
I do think Anker can improve but some of what you ask for will cause product failures ( heat = accelerated wear) and customer issues "it feels too hot" (like we see some complaints on the QC Powercore products).
Until Anker make it, then consider just multiple smaller charger, carry 2? 2 allows for a failure, which matters if on a longer roadtrip. Or go with the Anker Powerport 5, it doesn't jut out as its just a cable out the wall. I find the sweetspot is the Aukey (v.small where that matters) and the Anker Powerport5 (more Watts when that matters).