I would say the most generously pouched thongs I have would be from N2N and Andrew Christian, specifically the "Almost Naked" line. They don't necessarily *look* like super-large pouches when laid out on a table, but the fabric is very stretchy, so they both allow for quite a bit of movement...
Nope. Swimwear usually has much stronger elastic (so it doesn't peel off when you dive in, for example), and is made from materials that don't absorb water, so they can dry a lot faster than, say, cotton or the like. The fabric also tends to be heavier.