You've got it in your last statement, the larger diameter of the lp turbine on the d4 reduced the (effective) area of the cold stream duct.
Thats as I remember it anyway. I do remember it being a bit of squeeze to get in the duct compared to the 22b and 524b.
Anyway, whatever, still a damn fine engine!
Come to think of it I used to have the exact reason for the difference written in my old notes. One of those totally useless bits of info that one collects
However having stored them for years as loft insulation they got binned in a big clearout.
Maybe I could have sold'em
