Not sure, either they reduce leg space on the seats or because the seats are more compact which allows them to squeeze in an additional row of seats. Only way to find out is to compare 7777's length with the Wrights and CDGE.
Since we are talking about length, SMB388S is 17.95m, not 18.75m. Or else SMRT would have already achieved a total seating capacity of 60 seats by removing 1 PIW area.