Mode 7 scaling; that is, making Layer 1 shrink or grow. Its values are calculated and stored into the respective Mode 7 parameter mirrors at $7E:002E through $7E:0035.
The first byte - $7E:0038 - is used for horizontal scaling, whereas the second byte - $7E:0039 - is used for vertical scaling.
Default value is #$20. The closer to #$00, the more the layer grows, the further from #$00, the more the layer shrinks. Value #$10 makes the layer twice as large as value #$20, value #$40 makes the layer twice as small as value #$20, etc.