Simple. Lets say in a simplification that, hypothetically, Walmart employed 1 manager, and 20 employees. Lets say that they took up the niche that previously, 5 businesses had taken up. Those five businesses had one owner, who also worked as the store manager, and two employees each. That's roughly 15 people.
Those five managers would be what we called middle class, making between forty and seventy thousand a year. Their employees might make anywhere between twenty and thirty five thousand a year. Taking the lowest quote, that's forty thousand a year, five times, and twenty thousand a year, ten times. That comes out to 200,000 + 200,000, or 400,000 in employee pay.
The theoretical walmart would be at roughly forty thousand for one manager, and 15,000 a year for each employee. That's 40,000 + 300,000 coming to around 340,000 with more employees than those whome they put out of business. Further exacerbating the situation is the fact that Walmart will not hire most people full time, resulting in more people just barely getting by on the amount of money they make
What this translates to, more or less, is an overall impovershing of lower class workers, and lower middle class jobs. That in turn directly translates into less money put back into the economy to buy more products, resulting in still less jobs.
The example I used is very imprecise, but more or less reflects one of the reason big business' can be bad. Not ARE bad, but CAN BE bad. If a business behaved responsibly and concentrated less of the money at the top, but instead used extra money to incentivize improved quality, service, and work ethic, there would be NOTHING wrong with big business at all.