Walmart works hard to show off its green side, but environmental activists have been slamming the corporation for a litany of sins against Mother Nature—emitting millions of tons of CO2, encouraging waste by selling low-quality goods, and building warehouse-sized stores. Many of these criticisms are connected by a common thread: the retailer's mammoth size. Can such a massive corporation ever be truly "green"?

This week, the largest corporation in America itemized its environmental successes in its Global Responsibility Report. For example, in 2011, Walmart kept 80.9 percent of all of its waste out of landfills and increased its locally grown produce by 97 percent. Outside sources have noticed, as well: according to the EPA, Walmart is the third-largest U.S. corporate consumer of green power—renewable sources with particularly high environmental benefits.

Walmart recognizes that it is under the microscope. "We've set bold goals for ourselves, and government leaders, NGOs, and our customers are watching our progress," company chairman Rob Walton said at the company's Sustainability Milestone Meeting this week.

Despite its considerable efforts, the company does have a detrimental impact on the environment in some ways. According to the Sierra Club, one Walmart Supercenter uses the same amount of energy as 1,095 U.S. homes do in one day. In addition, Walmart says that its greenhouse gas footprint has grown by 13.8 percent since 2005. Likewise, the company's green power usage—which takes into account purchases from utilities, as well as on-site generation and the purchase of renewable energy certificates—accounts for only 28 percent of its electricity use, according to the EPA. Other retailers far outstrip that: All of the electricity used by Kohl's comes from green power, and Whole Foods Market uses or purchases 106 percent of its green power needs.

There are other green issues that environmental groups raise. The Institute for Local Self-Reliance (ILSR), one such group, has blasted the placement of stores on the outskirts of cities, requiring consumers to drive further to reach them. The retailer's massive grocery operation also promotes industrialized farming, they allege, and the Sierra Club has pointed out that stores' 24-hour operations make for more energy consumption.

Many of Walmart's critics accuse the store of "greenwashing"—cultivating an image of environmental responsibility to win consumers' favor.

"No amount of greenwash can conceal the fact that Walmart perpetuates an industrialized food system that diminishes our natural resources, causes excessive pollution, and forces smaller farmers and companies to get big or get out of business," says Wenonah Hauter, executive director of Food & Water Watch.

Like many businesses, Walmart certainly has room for improvement. But given its size and business identity as a retailer of low-priced goods, one wonders how far it can go. Industrially-produced organic milk will naturally be cheaper to produce and sell— especially to meet Walmart's demand—than that from a grass-fed, free-range cow on a small-scale farm.

"The argument that academics make, and that I'd make too, is Walmart is many times the size of Target or Costco. [Those stores are] big, but they're not of such a size that they're so incredibly influential," says Stacy Mitchell, senior researcher with ILSR.

That means that, if Target or Costco grew as large as Walmart, they might face the same backlash. After all, one reason why Walmart's greenhouse gas emissions have grown is that the company itself has grown. And the company points out that its sales and square footage growth have well outpaced its emission growth.

However, Walmart's sales growth didn't occur in a vacuum. Especially in tough economic times, many consumers rely on low-cost retailers.

Cost considerations aside, "we're just a consumer society. It's the way we are today," says John Byrd, a senior instructor in finance and managing for sustainability at the University of Colorado Denver's Business School.

Short of a massive societal shift, it seems that fully "fixing" Walmart's environmental problems without shrinking its business is a pipe dream.

But some argue that by virtue of its size, Walmart may be uniquely positioned to lead environmental efforts.

"There's some critical size after which companies actually can begin to commit resources to implementing green practices. We would criticize those big companies at our peril." says Byrd. Indeed, the store has partnered with the Environmental Defense Fund and the National Fish and Wildlife Foundation on some green projects. Byrd adds that the retailer could be uniquely positioned to shift the economics of being green.

"If they see something that they like, they change markets," says Byrd. One example he points to are natural gas-powered trucks. Walmart is considering adding these trucks to its fleet, and Byrd says if it orders hundreds or thousands of them, it will promote more corporations to use them. "Manufacturers will design them much better, and the prices of those vehicles will come down," he says.

Similarly, many consumers are more likely to go green when it's cost-effective. The need for cheap groceries can be more pressing than any environmental repercussions. But low prices can create a vicious cycle. Walmart, the employer of 2.1 million workers, has borne heavy criticism for its compensation policies, particularly its recent cuts to health coverage for some workers.

That kind of cut perpetuates consumption and waste, says Mitchell: "Many people feel so strapped that they feel they have to buy the six-dollar toaster, but part of the reason that so many people are in that situation has to do with the Walmart economy."