Rewrite each original sentence in order to make it easier to understand by non-native speakers of English. You can do so by replacing complex words with simpler synonyms (i.e. paraphrasing), deleting unimportant information (i.e. compression), and/or splitting a long complex sentence into several simpler ones. The final simplified sentences need to be grammatical, fluent, and retain the main ideas of their original counterparts without altering their meanings.

The aggregate income distribution is highly concentrated towards the top, with the top 6.37% earning roughly one third of all income, and those with upper-middle incomes control a large, though declining, share of the total earned income.
The total income distribution is more to the top. The top 6.37% earns about one third of all income. People with upper-middle incomes control a large, though declining, share of the total earned income.