California had only 95,000 people when it became a state. At the time of statehood,South Dakota had 350,000 people while North Dakota had 190,000 people. There were political and economic differences (and a rivalry) between the two, due to among other things the structure of the rail networks, from the beginning: https://time.com/4377423/dakota-north-south-history-two. Both were growing very rapidly, and the southern part of the Dakota territory had held a separate constitutional convention. Harrison reportedly shuffled the bills of statehood before signing so nobody knew which became a state first—a sign of the rivalry.

Idaho had 90,000 people, and Montana had 142,000 (and was expected to vote Democrat). Montana, Washington, Wyoming, and Idaho had been separate territories for 35-50 years. This was more than many of the more eastern states had at the time of statehood. (E.g. Tennessee became a state at 75,000 people.)

We think of these states as fungible “flyover country” today, but when they were created they had very distinct identities. (Wyoming for example was a highly progressive state that allowed women to vote 50 years before the 19th amendment.) The economies were tied closely to the land (mineral deposits here or there) and the shape of the country’s rail networks. The perfectly square boundaries are somewhat artificial, but the states themselves represented quite distinct population centers and were quite populous for the time.