For me, my phone is my internet and my go everywhere computer without it having to be tablet or laptop sized. I use my phone as a PHONE probably 5% of the time, and as a camera and computer the rest of the time. But I still want something that I can carry on my person. So, back when I was shopping phones, I got the 920 because of its camera and its size, but I would totally love something even bigger. I won't move up size-wise however, until the 1020 camera gets put into a 1520 size. I'm sure it's coming.
I wouldn't care about size as much if there were still phones with physical slide out keyboards like the LG Quantum had. I would not want something iPad sized as my phone, but 6" would totally work for me. Since I use my phone for photography and images so much, a bigger screen would be great for that.
To each their own. We should be thankful WP is soon to be available in so many sizes and is already available in so many camera options! For me, I don't want to lug around something huge (tablet) and a phone together, so a huge phone would be perfect. I can totally see people not wanting a large phone, especially if you talk on it a lot... I can only say, don't buy one. No need to knock other people's wants. There's plenty of small and medium WPs, so why not have large ones as well?
I'm certainly not trying to stir up the tank (because I generally love reading SwimSwim's board posts), but just giving my own little example of why I would love a bigger phone. I just don't get it when people wonder why, as though it's wrong to want something like a big phone or a physical keyboard (I see that a lot too). Who cares why?? Some people just want those things, just some like some people really want Instagram or feel they need NFC. Again, we should be thankful Nokia/MS is trying to cater to so many different desires. The alternative is potential customers going to Samsung/Android!