Most of my friends and family carry their phones in their front pockets, if they don't have them in their jacket pockets or their purses.
I haven't had to buy a new pair of jeans in a while, but all of my pants have front pockets that are either the same size or larger than the back pockets. I have an S21 Ultra with a case and I can sit down no problem with the phone in my front pocket. There is no way I'd keep it in my back pocket, it would stick out like a sore thumb and be an easy target for a pickpocket, plus there's no way I'm sitting my 200lb + ass on a $1500 device lol.
Either way, while a phone is a larger device, the keyfob seems to be more prone to accidental button presses. Maybe the reason for wanting a smaller fob is the idea that a smaller fob's buttons are less likely to be accidentally pressed? Or maybe it's just to manage the amount of bulk if the user doesn't want to remove anything attached to the keyfob (house keys, gym tag, air tag/tile, etc.).