Denver proper has a population of 600,000. The metropolitan area has a population of 2.5 million.
18,000 is about 3% of 600,000, and 0.7% of 2.5 million.
Mentor, Ohio has a population of 47,100. 2,800 showing up is 5% of the local population.
The West Chester number is impressive. The town has only 60,000 people. In California, where I have lived, the population is divided. The urban areas tend to vote demo, and the rural areas vote republican. There are a few well known conservative urban areas which are the exception.
If Romney went to Orange County CA, I would expect him to draw a huge crowd. If he went to a stadium in San Francisco, I would not expect that to happen. The converse for Obama.