That's an interesting point, and it's one that I'd argue is the opposite of what you'd think.
The guts of these devices are all surprisingly similar when it gets right down to it. Yeah, the big ticket items that we notice - CPU/GPU/storage - are all different, but the biggest thing that people notice, that the bottom of the phone gets hot, are all radios being activated. These radios across all phones are drawing very similar levels of voltage and thus will produce similar heat output.
That means that the phones that don't feel hot, are likely insulating the heat from the radios from your hand. Which is bad - you want the heat dissipated as quickly as possible. So, phones that feel hot may be the ones doing a better job at moving the heat away from the radios.
The are other possibilities, of course, like some phones may be designed to dissipate heat over a wider area, but it's unlikely on a complex device of this size that the heat is designed to go anywhere but right next to the radio and then out.