It depends on a few factors, primarily determined by choices made by the device maker, informed by the available technologies and their market targets.
I can’t speak to these later model iPhones (I’m typing this on an 8 Plus; for what it’s worth, I’ve found this one to be consistently cooler than my 6 before it, and my 5 before that; I was a luddite with a land line before that so I have no more anecdata), but generally speaking while iPhones can and do get warm, their thermals are incredibly impressive given the form factor. These are devices with passive cooling which are performance competitive with the MBP you mention, despite the engineering that went into its own active cooling system.
These devices make different tradeoffs, for different goals. But also importantly, they use different technologies. And with these specific devices in mind, that’s where “it depends” gets very speculative but also interesting at this moment.
I think it’s pretty likely that passively cooled handheld devices will continue to be designed by facing tough choices between performance and efficiency (heat budget, yes, but also power drain). Of course those tough choices, combined with growing expectations of what a handheld can do, are what have driven the impressive improvements in performance per efficiency in the iPhone line.
And Macs are about to transition to the same CPU/GPU platform. So “it depends” is really about to what degree Apple will use the performance per efficiency gains in that switch to prioritize:
- Performance overall
- Thermals and power consumption
- Form factor
And at this point the rumor mill is surprisingly quiet! It’s hard to say what balance they’ll strike, in which Mac products.
Speculating based on many years of being an Apple customer (as in I’m an Apple greybeard, I’ve had in my home or owned at least one Apple device for each major ISA they’ve shipped since the original Mac)... I think they’ll probably target all of the categories, but favor (in order of priority, highest to lowest):
- Form factor “improvements”: taking advantage of better thermals to make devices thinner/lighter/smaller/more visually striking
- Form factor improvements that are not controversial enough to warrant scare quotes or debate about whether it’s a genuine improvement: improved battery life, improved cooling (and they’ll probably focus at least the marketing on noise reduction)
- At least enough of a performance advantage to show (some cherry picked set of) CPU/GPU bound apps outperforming the same apps compiled for x64 on the previous gen Mac, and (also cherry picked) x64 apps performing competitively under emulation
In other words, I expect Apple to keep the same set of priorities they’ve had, for better and worse, since the PowerPC switch, replicated in the colorful plastic era, the Cube, the mini, the Intel switch, the introduction of the iPhone, the retina MBP, the trash tube.
tl;dr they’re probably gonna make some modest heat dissipation improvements, but prioritize other things higher, because they’ve very seldom deviated from that since the Pepsi guy left.
Edit: I honestly don’t remember my Apple lore well enough to remember if it was the Pepsi guy! I remember the product direction pretty well, but the human trivia isn’t my thing.