On of the reasons I'd like a smaller device, though there is the already stated compromises around smaller battery too, and it being a general use device. If I had a small device with excellent battery life I could carry that normally and tether a larger device, in my backpack when not in such use, for mapping and other when needed (I'm not talking nipping out for 10K here, sometimes this is full- or multi-day walking-or-faster events).
> having to stick a gigantic slab in my pocket … reminds me that I've got a brick flopping around in my shorts.
While the perfect phone doesn't exist, I have found the holy grail of shorts: big enough pockets that the slab fits, but tight enough and small enough that it doesn't jiggle noticeably, but not tight to me such that is exacerbates sweat in that patch. Also, the phone for nav on long routes is tertiary, I have breadcrumb trail on my wrist and a printed map (on rip- & water-resistant paper), so if I'm going far enough to require a bag for water/foot/1st-aid/other then there is room for the slab in there too and it isn't too out of reach.