It seems plausible to me that when an inside lawyer has to orchestrate several firms and vendors on a project – create a virtual team – that lawyer would have bandwidth to manage fewer cases than a counterpart who hires a firm to do it all (See my post of Jan. 10, 2010: a virtual firm for Pfizer; and Aug. 10, 2007: virtual firms with 6 references.).
A speaker at a recent Ark conference suggested that the constraint is not so much the managerial ability of the inside lawyer nor the number of firms or providers managed but the characteristics of the entities. Good ones collaborate well, understand and accept their roles, and minimize managerial demands. The extra burden that you might expect may barely happen.