They made the enclosure thinner. They also changed the fan design and used a completely different set of major components. The new components have different heat tolerances than the old ones, and they generate different amounts of heat. In other words, everything changed, NOT just the size of the enclosure. How this stuff all works together is "thermal engineering". (Reading system temps is not what I call thermal engineering.)
"Run hot" is a subjective and nearly useless concept. Hot compared to what? Absolute zero? Room temperature? Body temperature? The boiling point of water? The core temperature of the sun?
What matters is HOW HOT a component is for HOW LONG, relative to data about the MTBF (mean time between failures) for the component at various temperatures. Is 40 degrees C (to pick a random number) "too hot" for a CPU package? It depends... what is the MTBF at 40 degrees? At 35 degrees, or 45 or 60 or whatever? Beyond the simple "cooler lasts longer" or "thinner is hotter", real engineers have to deal with a whole basket of tradeoffs. Component lifetime vs. noise levels vs. case size and shape vs. performance vs. cost vs. fan speed and placement vs. baffle design, etc. etc. etc. The goal is rarely ZERO component failures, or ZERO noise, or any other extreme. The whole story is about what tradeoffs are possible, and with what expected consequences. That is "thermal engineering".
I don't think anyone outside of Apple has access to the data needed to judge whether this thermal design is good or bad.