I have the Galaxy Tab, the 7 inch one that came out couple of years ago. Android, so it matches my phone apps.. but here's the catch.. I can actually SEE it.
My bet for why tablet owners buy more is that tech savvy near-seniors can't see crud on our phones, but we're don't have to run home to the pc or call our kids to order things for us. If I get a Groupon while I'm out, buying on the tab is easy, and I won't forget to do it at the house. If I have to use the phone, forget it.
I use the phone for making phone calls and texts. e-mail and app use on the tablet. I expect larger screen mobiles will get more traction as the population ages.
By the way, when I'm out, I can show potential customers pics of my product line in a much better size, that has increased sales a lot.