In large biome worlds, the biome are scaled by 4 times large. However, the rivers are not scaled.

The screenshots are taken in the world "3116913" in 1.18-pre4 near (0, 0) at (+x, +z). The map area in "4x" are 2048x2048, and in "2x" in 512x512.

The river pattern are the same in "large 4x" and "normal 4x", which shows that the rivers are not scaled.