We chose 25' trailer so we could access every national and state park we encounter. Cades Cove in Smoky Mountains has some small sites - we backed our 25' Airstream into a tent site no problem. Like David says, we're just closer to the outdoors lots of times. Sometimes we arrive an old national park campground and the available sites are the smaller ones. Again, no problem for our trailer.
We love sleeping in our own bed on our own pillows every night. We love the freedom of hitching up and leaving whenever we please. We like the relative simplicity of life when all your "stuff" is in the trailer or in the truck. Storage in the International CCD25 Sleep Six models is huge. That works well for us, although we sometimes seem to misplace things still. Go figure?
We like being energy independent and having enough tanks capacity to go fully 8 to 10 days with no hookups of any kind. A pair of solar panels and a 1KW genset help with that, as does learning to really scrimp on water usage.