I rented a Peugeot 108 from one of the major chains at Belfast City Airport a few years ago (possibly pre-Panda, in fact). What I remember about it was quite how visible the cost cutting was in the design. I remember when launched how the 108 / C1 / Aygo was described as a groundbreaking design in that sense (with no separate panel between the fuel filler lid / boot lid / rear door for instance) but the overall result was that the interior - especially - felt really tinny and cheap.
One of the great characteristics of the Panda is how, when driven, it doesn't feel small or cheap. Sure, if you frequently jump in and out of bigger or more expensive cars, you'll notice it. But as my daily driver, I enjoy the fact that everything I touch or can see from the driver's seat feels solid.
Compare the design of the interior of the C1 and the Panda, and you'll notice a clever trick Fiat's designers took with the Panda. By emphasising the horizontal width of the car with the coloured panel that loops around the dashboard fascia, you get a much more generous impression of the car's size than in the first generation C1, where the single little pod-binnacle only emphasises how small the cabin is.
Hard to explain, perhaps, but I think they're two cars that make a good comparison of why the Panda feels bigger.