For example, suppose that the BBG is a Fighter that uses an oversized (Large) Estoc (threat range 18-20) and has taken Improved Critical to make it 15-20/x2. Entering said BBG's appropriately ominous oversized room, the party notices what appears to be a Gargantuan sword in the corner. Combat starts. BBG uses an item that gives him Giant Form II, as a Storm Giant.
BBG is now Huge with the ability to pick up the Gargantuan Estoc and wield it as long as he uses 2 hands. AFAIK his threat range is still 15-20 but the Estoc's point is likely nearly the size of a medium creature's torso.
A similar dissonance exists between Medium creatures and Tiny or smaller creatures as well as an inverse when a tiny creature attempts to perform a devastating attack on something magnitudes of size larger than themself.
My question is, are there any rules that affect your ability to land critical threats when the difference between size of weapon and target are so exaggerated?