KL863 AMS-NRT on OCT18 flew 11,834km for 13h6m.
KL864 NRT-AMS on OCT19 flew 11,992km for 13h36m.

Not much difference in terms of distance and time.
With the tail wind flying eastward, less fuel would be used as well.

For reference, KL868 ICN-AMS on OCT17 flew 10,405km westward for 13h25m.
The arrival time whether flying east or west will not differ much.

SQ23 and SQ24 between SIN and JFK and vice versa both always fly eastward as well.
For such long routes, it is almost always more cost effective to fly east, especially during winter.