This would be Bozeman / Yellowstone airport in Montana.
If we think PWM has grown a lot in the past few years....BZN has exploded. Its annual passenger numbers have almost doubled in a few years and it now has seasonal service to multiple east coast cities.
The vast majority of BZN's growth has been fueled by leisure travel, but BZN has a bit more "year round" tourism appeal than Portland (particularly as a gateway for winter skiing)
You would be correct on this one! This is my hometown airport. Bozeman has a population just under 15% that of the Portland metro area yet last year total passengers served just exceeded PWM at 2.26 million. For a city of it's size it probably has the best air service in the country if you can get a seat because flights are almost always full which makes it difficult for myself sometimes because though I fly for free I fly on standby.
Outdoor recreation/tourism drives the growth of Bozeman and the airport is the main stepping off point for Yellowstone Park. BZN puts PWM to shame when it comes to amenities on site. Every time I go home I am in awe of how much the airport has changed as it is under constant construction as is the entire Bozeman region. It is no longer the beautiful small town I grew up in. The local university (Montana State - Go Cats!) also makes USM look like a high school.