I've had the Plus sizes since they were first introduced. I carry my phone in my front left pocket. I'm 5'10", athletic/muscular, but I prefer not to wear tight pants just because I don't like them. I've never carried a phone in my rear pocket -- the thought of sitting on my iPhone has never been appealing.
On rare occasion, I'll have to shift the top of the iPhone to the 'outside' edge of the pocket when I'm bending over (eg. while sitting to tie my shoes), to make things more comfy and to prevent possible damage to the phone, but other than that it works fine in that pocket.