We need to remember that the largest iPhone screen was 3.5 inch in 2010. So, using your analogy, it was more like going from a 13.3 in laptop screen to a desktop with a 24 inch monitor. There are some things that are just painful with less screen real estate. If screen size didn‘t add useful functionality, people would not invest hundreds of dollars in external displays.
The original iPad was a response to the popularity of netbooks. It was designed to address many routine tasks: mail, web browsing, photos, video, calendar, contacts, and books. Apple also created iWork apps especially for iPad. Frankly, the productivity experience was not great, but just by virtue of the extra screen size, you could write a simple letter or memo on the iPad that would be painful or impossible to accomplish on a phone.