And how does this relate to a 13" tablet?
10" tablets (like my iPad) are fine, but too small for reading regular PDFs, or for browsing most magazines (both PDF or from services like Zinio). Hell, they are too small even for browsing the web comfortably.
7" tablets are useless (Steve Jobs said it). Too small for anything useful, on the market mainly because they are cheap.
Because unless they make it out of Apple Magic Metal & Apple Magic Glass, made from sand on Mars that makes it weigh 1/4th as much the current iPad, a 13" iPad will weigh even more and be even bulkier than the current iPad.
Count up all the postings by people who say that the current iPad is too heavy for long term practical use.
The Laptop example was a representation of how size correlates to weight, in most cases, and how people think they want something until they experience the reality of it.
Here's another example: People think they want windmills for electricity. They're beautiful (in the distance), and green, and make you feel oh-so-good that you are saving the world from filthy oil.
Then they experience the reality of electrical windmills: How they are 400+ foot tall monstrosities that make a constant, low frequency, whup-whup-whup sound, that they are filled with oil-based products to make them run, that there are huge numbers of environmental factors, etc. etc. etc.
Suddenly the reality is not living up to the perception....
Make your own 13" iPad: Find a way to increase the weight by about 1/3, and then try carrying that around. How much do you like it?