Why do the rural poor of Montana matter more than the urban poor of Chicago?
They matter the same. That is why it is better that the poor of Montana get to set their own policies. Otherwise, they have no say in anything.
Under my proposal they would be equally represented.
Not really. They would have different needs that are completely unrelated, and one group would have a lot more votes than the other.
There are plenty of underrepresented groups in urban areas with completely different and unrelated needs, too.
Like who?
gays immediately come to mind. Various immigrant groups, industries, and ages of people are often underrepresented in some form.
Do you think somebody living 400 miles away from a ranch should have equal say on the environmental policy the ranch has to follow as the people living in close proximity?
Yes, because environmental policy affects everyone.
However, new state lines would create larger blocs of people with similar interests. A great example of this is counties in New York that were proposing secession to Pennsylvania to get around the fracking ban. In my concept, they'd already be grouped with rural Pennsylvania and wouldn't be beholden to them whims of New York City.
So in your Montana example, it could have a negative affect on representation, but it would give them a larger pool of state resources to draw from. And it would increase the political power of rural folks in Colorado who had previously been dominated by Denver and Colorado Springs.