That's not a small town... the definition of city vs. town varies from state to state. By Colorado definition (the state I've lived in all my life, and haven't just visited mind you, so I do know what I'm talking about) Boulder is classified as a city.
Here's a list of major towns vs. cities in Colorado, and you can see that it's clearly a city by Colorado law.
http://www.citypopulation.de/USA-Colorado.html
Boulder has the 11th largest city population in Denver, which is quite a bit more than a "small town". Regardless of what YOUR opinion of what a town or city should be, it's a city.
I honestly don't have a standard definition that would be related to population sizes. My hometown is 450-500k (Raleigh, NC). I'd make fun of someone who considered that a city. It...."feels" like a town, b/c it's basically just one large suburb after another. All of RTP is above 1 million, but it's very spread-out. Durham is the only spot that has that "city-like" downtown.
Charlotte is a city, ~1 million, but it's still very suburby. it does have a well-defined downtown area.
Although things are changing rapidly, there is no real downtown--certainly not when I was living there. There is no effective public transportation. There is a beltway and shitty traffic, but in my mind, it still has that small town feel. I'd honestly think that a place like Berkeley is more of a city than Raleigh, despite the drastic difference in population--Oakland, El Cerrito, Richmond, and SF of course--very large metro area that feels like one city, more or less.
To me population makes less sense than does development and access. Yeah, it's a very personal approach, with strange logic perhaps, but I do think that resources and especially well-connected public transport are essential for a real city. High density residential areas, as well.