EDIT: I also get the feeling that bigger phone means more features and longer battery times because of the extra space for electronics and the battery. Who knows, maybe if I held a 4" that's fast with a lot of memory and long battery life, I could get accustomed to seeing less content.
There's also the fact that website layout is determined by the website developers. I wonder how good the layouts are generally on the internet for 4" screens. It might be that many websites have fixed width side margins that cause a bigger whitespace-to-text ratio on smaller screens.