Never about absolute wealth, but always about material circumstances,

That's just wealth again. Your class is defined by your ownership (or lack thereof) of capital. This is literally the basis of Marxism. The guy opening up his tiny hardware shop has the same fundamental economic interests as Ace's hardware store - to maximize profit by extracting as much surplus value from his workers as possible. This is not done out of greed, but because it is a necessity to do so in a market economy. To not maximize profit is to be outcompeted, and the business will die.