it is a bad fit for your hand if you're a normal human
I am over 6 feet and I can't hit the corners of my nexus 5 with a single hand. I've had to rearrange app icons on my home screen to be able to switch between spotify and google maps while driving. For a 5' tall person probably the top inch of the phone -- notably where the status screen comes from -- is inaccessible with one hand.
Personally, I'll never use a smaller phone again. I use a lot less of my tablet since getting my nexus 5; in practice, it's big enough that reading on it doesn't suck.
Please - don't hold your phone while driving. It's incredibly dangerous for you and other people on the road.
If you absolutely have to interact with your phone while also steering a couple of tonnes of metal around, please spend a couple of bucks on a dashboard holder.
They were not defending small screen. They were basically saying big screen is useless, stupid, and bad fit for your hand if you are normal human.
Oh well.