JCPenny, the company with the highest ratio on the first chart appears to employ about 117,000 people[0]. If the CEO gave up all his pay, they would be able to hire 1,795 more people - about 1.5%. Add together the top 20 and it's 18,860 people. Those same numbers likely apply to how many low-paid employees could be increased to around $15/hour[1].
In absolute terms, those sound like big numbers, but the US officially[2] has 9.8 million unemployed people[3] and a great many more working for low pay. 18860 more minimum wage jobs would be less than 0.2%. Flattening out the pay distribution in large companies may appeal to our sense of fairness, but it doesn't actually do much to alleviate poverty.
[0] http://www.macroaxis.com/invest/ratio/JCP--Number_of_Employe...
[1] The underlying assumption being that most of these companies have a large number of minimum wage employees.
[2] Unemployment numbers are skewed low because they don't count people who aren't following a specific pattern of job-seeking.
[3] http://www.bls.gov/news.release/empsit.nr0.htm