Propane has three carbon atoms lined up in a row. The two at the ends hold on to three hydrogen atoms each, but the one in the middle can only hold two hydrogens. Propane is heavier than methane and ethane. Some cars and trucks run on propane. If a molecule has more than three carbon atoms, it gets slower and heavier.
Butane has four carbon molecules, so it's heavier than propane.
In the summertime, we can use butane instead of propane, but in Alberta winters it's just not a gas anymore. We almost always sell methane as a gas, and send it through natural gas pipelines. Ethane, propane and butane are often squeezed until they become liquids so they take up less space and can be driven around in tanker trucks. When they are like this, oilmen call the natural gas liquids. Hydrocarbons with more than four carbon atoms are too heavy to be gasses.