The PMy is definitely a 'day-pack' holds an umbrella, sunglasses, a small case with my contact solution/toothbrush and room for some other small items. It would easily hold my phone (i have an iphone pro but a promax would fit fine) but way too small for an ipad. perfect bag for weekends/going out on the town etc.
The GM feels and looks bigger. It can hold my laptop, cords and everything else in that I transfer from the PM to the GM. My regular backpack is heavy to the GM is heavy when i use it but i don't find the single strap any heavier or more annoying than a regular backpak.
I am travelling now but will take some photos of both on me and send when i return home later this week. I did the same as you - snagged it from hermes.com to try and then ended up keeping it.
I even had it painted my maxpaintsbags (he does custom hermes bag painting) and love it
Let me know how you make out.