If they were defined by percentiles (i.e. certain definitions of middle class dictate the middle 60% of the population by income), you could still describe a "g

If they were defined by percentiles (i.e. certain definitions of middle class dictate the middle 60% of the population by income), you could still describe a "growing middle class" as a middle class acquiring a greater percentage of total income. While the population of the middle class isn't growing, its buying power still could be.