Let's put this in a little perspective, shall we. Pizza sizes are by diameter. Since the area covered by a circle equals pi time the square of the radius, the total size of a pizza increases by the square of changes to the radius.
Typical "pizza restaurant" sizes are 6-8" personal, 10-12" small/medium, 14" large, and 16" extra large. Take-out pizza is pretty much $15 for that extra-large size (for plain cheese).
In terms of area, it's 201 square inches. A 20" pizza, by contrast, is 314 square inches in area, or 56% larger. Proportionally scaling the price gives me about $23.50. And that's "plain". Add 3 toppings and we'd be looking at closer to $28-$30.
Now that's for a generic, chain, take-out or delivery pizza. For a nicer pie at a sit down restaurant, I'd expect to pay more. The OP was talking about a "plain" pizza, but the price range for a basic margherita or the most heavily topped pie in the place is only $2, so it may make more sense to compare to specialty pizzas at $36 rather than "plain" pizzas at $34.
based on that, I fail to see a significantly worse price premium for such a pizza at Via Napoli than I see at other Disney restaurants.
And yes, I'm a geek. I know it and accept it. (Which is what makes me a geek, as opposed to just a nerd.)