are you referring to reducing the size of the high-leg conductor ?
They key is that the conductor must still be sized to the load and protected against over-current.
In this area when the high leg systems were common, you would often find a 3 pole fusible disconnect with one smaller fuse to protect the high-leg.
Generally I would say that down sizing the high leg might not be a good idea as you already have to determine the line to neutral loads supplied by the center tapped transformer to prevent over loading it, 4-wire deltas come in two flavors, one with an over sized center tapped transformer for more line to neutral loads and few three phase loads, or smaller center tapped transformer for less line to neutral loads with more 3-phase loads, the load on the high leg will depend upon how much 3-phase loads you have and in addition what line to line single phase loads that might be connected to the high leg, an open delta (two tank bank) can make this even more of a problem as for having a balanced system.
In my experience I have never seen the high leg ever down sized, as it would be like having a forced unbalanced system, nor do I know of anything in the NEC that would allow it like it does for a reduced neutral?