Contract workers also saw gains. Survey Monkey, under pressure from employees, now offers full benefits to its contractors. Microsoft requires subcontractors with more than 50 employees to provide paid family leave.

Over the last year, 18 other large employers updated their paid leave policies, with many of them, like H&M and Dollar General, closing the gaps between hourly and salaried workers, according to Annie Sartor, an advocacy director at PL+US, an advocacy organization that worked on both the Starbucks and Walmart campaigns.

The spoils of the slowly improving U.S. economy have gone mostly to the highest earners, and a competitive labor market at the highest end of the earnings spectrum has led to a benefits boom. In the last few years, in-demand workers have seen longer paid leave times, coverage for fertility treatments and egg freezing, and employer sponsored student debt repayment plans.

Less than 10 percent of the companies surveyed by the International Foundation of Employee Benefit Plans extend these benefits to all of their workers. Contract workers, who make up 10 percent of the workforce, are left out of the statistics altogether and often get no benefits at all.

Companies have historically suggested part-time or hourly workers don't need full benefits: They're students, teenagers, or stay-at-home parents working to make extra cash on the side. That logic no longer holds. “A lot of people who work at H&M are parents and need time to take care of their kids,” said Artavia Milliam, an employee at H&M who helped negotiate more benefits for her part-time colleagues.

People who work more than one job make up 5.1 percent the workforce, a figure that has held steady even as the economy has recovered from the recession. The trend suggests that in parts of the economy, full-time positions have been permanently replaced with part-time workers, who generally earn less and have less stability.

Extending benefits to all workers can get expensive. “There are up-front costs in doing this,” said Rent the Runway's Hyman. Employers spend almost $30,000 per year, per employee with a family of four, for employer-sponsored health insurance.

Hyman believes the added costs will pay off. For one, better benefits will increase retention and lead to savings on training and hiring, she says. And because hourly workers didn't have as much flexibility or paid time off, some have had to quit in order to take care of sick family members. “Can you imagine a senior executive at a Fortune 100 company having to choose between grieving for a loved one in their family and keeping their job?” Hyman says.

She also plans on using her own company as a case study to track the potential cost savings. She claims loyalty rates and productivity are already up. “We can debunk those myths that this is just a sunk cost,” Hyman says. “It's not. This is an investment in your people, your talent, your culture.”