Warpage depends on a lot more factors than size. The biggest thing to pay attention to is moisture content, because wood changes shape and size as it absorbs or looses moisture. If your lumber is acclimated to normal indoor conditions before you build anything out of it, it'll stand a better chance of remaining fairly stable.

That said, it looks like you used just about the poorest quality wood available, and chose some marginal construction techniques. That cheap, knotty, home-center pine is not great stuff, and mitering the corners on such wide pieces is just begging for the thing to disassemble itself.

Thanks for your feedback Jon. I used knotty pine since our cabin in Maine has several walls of knotty pine done about 9 years ago. No big box stores within 50 miles of here so it's from a lumber yard and was extra so it's well acclimated. The idea was to have a casual look in tune with the rest of the place. It will be interesting to see how it winters over.