The current Hilux is extremely close in size to the US Tacoma... It has also grown over the years. Although if you look at the footprint size (e.g. what CAFE measures), the Tacoma has the wheels a little more advantageously placed.

I am sure they could consolidate the models to work in both the US and abroad, but my guess is they do enough US volume that it is not yet advantageous to do so. There's already a number of major parts that have been shared recently between the Tacoma and Hilux... e.g. the 2TR-FE engine and AC60 transmission. But usually Toyota chooses to spec the Tacoma as a more up-market vehicle, which makes sense given the US market.