One of those rare moments when I do self promotion for my post which contains the argument about the shrinking middle class with the materials used were based from Indonesia's data. I highly recommend reading about wealth inequality in wealtheconomics.org by Gary Stevenson as this is the current working theory I subscribe in. The entire site can be speed read in under 30 mins including the other blogs within it because it has already all the gist you would find in his youtube videos explaining the phenomenon.