> I also have an iPhone, but be fucked if I can keep hold of it, it's so light I can seem to actually get a grip on it for longer than about two minutes at a time.

Your half-kg phone has trained your motor neurons in such a way that they are not prepared when you hold a ~100g phone. I suggest mixed-interval training. Use both phones regularly (you mention the half-kg one is your daily driver). An anti-slip silicone case for your iPhone should also help.

When I hold an iPhone, it sometimes feels heavy to me. I would say my experience is more representative of the general population than yours.