Large firms in the drug industry such as Merck (market cap of $193 billion, Beta of 0.80), Eli Lilly (market cap of $252 billion, Beta of 0.75), and Pfizer (market cap of $343 billion, Beta of 0.80) all have a past record of high profit margins and high return on equity (ROE or net income divided by book value of common equity). Despite sustained high levels of profits these firms maintain rather low levels of debt, e.g., ratios of debt to equity of 0.09 to 0.12. Thus these large and profitable firms in the drug industry might appear to have the capacity to carry larger levels of debt, but they instead carry low levels of debt. Meanwhile, smaller firms in the drug industry such as Teva, Bausch, Biogen, and Perrigo have considerably higher levels of debt than the larger firms, have higher equity Betas (usually higher than 1.0), and while profitable they don't match the profitablity levels of the large firms cited above. These two categories of drug industry firms clearly have different capital structures.

How can one explain the sharp difference in capital structure between the large and small drug industry firms?