According to the Book

The book is called "How the States Got Their Shapes" by Mark Stein, published in 2008.

A recurring theme in the book is that Congress usually tried to make all new states approximately the same size, but geographical and political considerations frequently interfered with that effort.

Before Minnesota became a state, its territorial western border was the Continental Divide. That would have made it way too big to be state so, when it did become a state in 1858, the western part was chopped off and organized as Dakota Territory. In 1863, Congress created Idaho Territory in response to gold being discovered in the region's mountains. This infringed on Dakota Territory, leaving it the size of present day North and South Dakota combined. The two Dakotas were divided in 1887 because Congress wanted to make two states that were each the approximate size of Kansas and Nebraska.

I was going to write about Idaho tonight, but it's kind of a long story, and it has gotten late on me. I don't know how that happened, probably some kind of temporal anomaly slipped into the time/space continuum. I'll do Idaho tomorrow.