Tire size

I bought a 2008 Highlander a few years ago that was extremely well taken care of and only had around 88 K miles on it. The only thing that was odd about it was the tires on the vehicle were Michelin 255/60/19 tires. The vehicle stock tires are suppose to be 245/55/19. I can not think of a reason the previous owner would have put those tires on the vehicle except that they fill the wheel well a bit more. Now it's time to replace them and not sure what to go with, the stock size or the bigger size...any thoughts ???