It might just be me, but I would never feel safe keeping my piggies outside. For one, you might not see them as often as you would with their cage right near you all of the time, and would therefore be less likely to notice signs of sickness or pain if your piggies were to catch something. In addition, all kinds of predators such as snakes, cats, dogs, and even people would have a chance of getting to the pigs, no matter how small it was. Yes, I have heard of many happy piggies living outside their whole lives, but what if yours are in the few percentages that are injured or killed because of their living outdoors?
You are definitely not being selfish. A 2x4ft cage is actually the size recommended for two sows, though I must say I can't blame you for wanting to give them more room.
If you do feel that you really want to get a bigger cage, C&C cages are a great option-they are generally very affordable, and materials, depending on what you are using, could end up being very low-cost. Is it possible for you to move furniture around to give them a slightly bigger space to fit a larger cage? Usually, with just a quick look around, you can move an old bookshelf or scoot something over a few centimeters or so to make more room.
I hope this helped, and good luck with getting your piggies' living space sorted out!