So you're trying to compare the Net welth of ~750 individuals to the anual spending habits of ~120 million people (40% of american population). Lets dig a little deeper.
Using your numbers from above the Avg net worth of these 750 billionairs is ~$1.87 billion. Now assuming $5 trillion spending each year the Avg spending per middle class individual is ~$41,667 per year. Multiply this out over a lifetime (72 yrs) and that individual spends ~$3 Million aka $0.003 Billion. Now I realize that spending does not equate to earnings, but I think you would agree that most middle class individuals spend close to what they earn in a livetime. However, just to be generous I'll assume that the middle class has a 25% savings rate(LOL). Tack that on to 3 Million and you get $3.75 Million ($0.00375 Billion).
Now I'm not sure what numeric system you're using, but looking at these numbers the "normal" wealth of the middle class is nowhere neer the wealth of these Billionaires over a lifetime.