Actually I can type much faster on the smaller iPhones than the big ones simply because the keyboard letters are closer together so my finger doesn't have as far a distance to move between keys. I type one handed using hunt & peck with my index finger and when I'm on a roll, I can fly through a long page of text almost like typing on a laptop.
You wouldn't think that having the keys farther apart on a bigger keyboard would slow down the typing, but for me it definitely does. It's a looong finger reach to get from the "a" to the "p".
But that's just speed. I prefer to use the big iPhones for everything except carrying in a shirt pocket.