If you use Aggr(sum(UnitSales),Product)), you will get only one value of UnitSales by each one Product.
If you want to get a table with Product Dimmention and only one Expression with the top UnitSales Customer:
For your desired result, consider Anil Babu's answer. You aren't getting the desired result as the aggr function is giving one value, which is again been ranked on the basis of Product which effectively remains same, as you have already grouped by Product, so in any case, you'll be getting only one value.
Hope it helps.