While I understand and agree it's easier and more productive to work on a large screen, I feel there are advantages to the smaller screen size.
The primary is that it forces you to keep information in your working memory. So you'll either improve your mental capacity, or use better abstractions and architectures to be able to hold them in your working memory.
But once you accomplish that, it is more comfortable to go back to the bigger display :)
I do almost all of my work on a 13" MBA. Works out fine. I do have a very large monitor that I can use as needed though. Except for Java, I use terse programming languages, so functions/methods are short, and I don't need to look at a ton of code at any given time. A few small edit windows open and visible does it for me (mostly tear off windows from IntelliJ).
The primary is that it forces you to keep information in your working memory. So you'll either improve your mental capacity, or use better abstractions and architectures to be able to hold them in your working memory.
But once you accomplish that, it is more comfortable to go back to the bigger display :)