There is no surprise Apple has changed the way designers were trying to create the look of portable computers before iPad appeared — just like the iPhone has changed the whole understanding of smartphone design.
Nick Bilton from the New York Times has said that the earliest iPad models have appeared in 2000 and were 7-inched, just like iPad mini. But, whatever the size was, Steve Jobs did not like the first prototypes. Jobs has stopped a huge amount of early development processes when the devices were ready for the mass production and made the engineers do all again. In Bilton's opinion, Jobs just could not imagine such a massive, uncomfortable and energy-consuming device in customers hands.
At the All Things D conference, Steve Jobs has said that Apple started the work on tablet earlier than on a phone: