Well, it could if it meant “near median income” if few had near the median income. Median doesn't mean particularly common (or even the most common, which is the mode.)
|...|...........|...|
^ ^ ^
| median |
(working) (upper)
|.......|...|.......|
^ ^ ^
| median |
(working) (upper)
|.............|...|.|
(working) ^ ^ ^
median | (upper)
middleIt doesn't force the working and upper class to be equally large unless you further assume the distribution is symmetric, but yes, there is a limit to how small the upper class can be since at least half of the distribution has to be split between the upper and middle class.
[0] http://www.pewresearch.org/fact-tank/2018/09/06/the-american...
Currently in developed countries it happens to be the case that the people with median income are middle class. However this was not always the case, ie the group referred to as middle class did not always include people with median income and so the above definition does not coincide with what middle class typically referred to.
According to this [0] article from 2017 only 33.4% of Americans over the age of 25 have finished their undergraduate education. As such, MOST people are still without undergraduate education, and thus a median American does not have undergrad education. Since middle class should represent a median resident, it would follow that people without undergrad education should be quite solidly in the middle class.
[0] https://thehill.com/homenews/state-watch/326995-census-more-...