So, I bought a 3200Mah battery for my old Xperia X10. A year ago they were expensive things to buy from Mugen, but now they're 12 bucks on eBay.The battery is essentially two of the original sized ones stuck together, and it is supplied with a replacement backplate so it all fits. Lots of air inside that case. If it was more precisely designed, you'd have a 4000+mah battery under there.
With all the talk of super slim Razr phones, my phone is now an 18mm thick monster. And that's fine. If this lasts me a couple of days then I'm happy to have the extra lumpiness.
I reckon this should be a common option with phones. It's done with Laptops all the time. The choice to buy a big fat battery, accepting the loss of thinness. The new skinny beasts could gain a few millimetres and still be comfortable.