Do you remember the retina display on iPhone 4? Since then, phone makers seem to have been aiming to beat it, with higher and higher densities.
Cast your mind back further: the first iPhone. 'We got rid of all the buttons and just made a giant screen'. 3.5 was massive.
To beat it, the Samsung Galaxy S had a 4 inch screen.
Then, to ensure the phone looked better even to the casual observer, the next one was 4.3 inches.
Then the S3 which was 4.8 inches. Then, the S4 with a 5 inch display. And so on...
Screen size isn't always 'progress' sometimes it is a design choice. Samsung never made that choice, it evolved by accident.
Samsung's jump into a phone with a massive screen was the Galaxy Tab. Which at the time was the only Android competitor to... the iPad
Source:
http://www.techradar.com/reviews/pc-mac/tablets/samsung-galaxy-tab-903545/review