Most people have differently sized nostrils. It's an interesting physical difference, and there are a few different ideas for why. One is the nasal cycle, and your body simply has a preference for one side, another that you'll hear is (and this is constantly parroted in the EMS world and easily dismissed by the logic that your nose is quite far from the carina where the primary bronchi merge) that your lungs are slightly different due to the heart sitting partially to the left of your chest (for most people), making your right lung bigger and thus blows harder through the right nostril.