Actually "middle class" was first used in the 1700s to denote the class between the peasants and the nobles -- i.e. the merchants or bourgeoisie.
As opposed to it being the "normal" experience in the strict normal distributional terms, the reality is (a) income and social status is not a normal distribution -- it is extremely right skewed and (b) given some concepts of class divisions (e.g. peasants, merchants, nobles in previous centuries or blue collar, white collar professionals, and the rick (i.e. those who's earnings from capital returns as opposed to salary) today) middle class has tended to represent the group in the middle (as opposed to the person in the middle.
Thus, even if the bottom group of poor people is very very large, the middle class is not the mean of the population.
This is extremely clear on our usage when we say things like "Guatemala has a very small middle class as it is mostly poor people and some old money rich people."