That's not a correct comparison. You don't have to put your 30" monitor in your pocket or carry it around one handed. The monitor sits on your desk.
I wouldn't mind if the iPhone 5 was a 4.8" device, but I can appreciate its compact form factor at 4"
Furthermore, the increase in size of LCD monitors is dictated by manufacturing improvements. Back when they could only make small panels, a 15" monitor could cost you $800. I remember my mom wasting that much money on a monitor back then. I got a top of the line CRT for like $200-$300 for a 17" Beautiful 120 Hz display and stuff.
Then years later I bought her a Dell 2405 for like $600? It's like dirt cheap now. LCD monitors have gotten bigger because of cost reductions.
The same is happening with phones because pixel density is so high it costs quite a bit. But as that starts to improve, it seems we can then justify largeass screens. But yes, we do forget people want these in their pockets. Or to fit comfortably in one hand.
Here's an issue I'd like to bring up. Although I can hold an SGS3 ok, I feel like manipulating it is difficult. I was riding in the back of a scooter once and I whipped out my Nexus S and snapped beautiful photos and video. The larger the phone, the harder it is to press the power button, then swipe, then go to camera, rotate my phone to landscape, etc. I feel like it's easier to drop because it doesn't fit comfortably in my hand as much. There's more motion to shift to the upper and lower halves of the phone depending on what you're doing. Meanwhile on smaller phones you can just handle the phone as is without doing any shifting. And having medium sized hands puts me at average.
I foresee a mad rush to large and crazy screens, but then phone manufacturers will settle down and choose ergonomic sizes in the end and have a cleaned up lineup. It's just like the mad rush for phones and people released phones right and left, but realized that a condensed lineup is all that's needed. Well here we are too. There's going to be a limit reached soon for this mad rush for the largest screen.