There's another surprising reason why per capita numbers aren't useful - for exponential growth on the typical type of graph starting at some "initial" number of cases, it makes no difference! For example, if one country was counted a two equal half-sized countries, their graphs would be the same shape but shifted to the right by a few days. However, they would also reach their "initial" number of cases where the graphs start at a few days later - shifting them left by the same amount! The result would be the same line as the full-sized country.