Just a guess on why . . .
To maximize the sweep coverage, as the 2 arcs work "best" with difference sizes. That way, more of the windscreen is cleared w/ each sweep at minimal mechanical complication. -For the opposite, complex mechanics, look at the variable length arms on Mercedes. They physically shorten to clear the edges of the windscreen. On my Miata, I installed ones larger than stock, but they loose contact on the far edge of the screen- which doesn't bother me in the slightest. The result is a larger cleared area when running the wipers.
Scion/Toyota would have used 2 sizes and spaced the motor axles accordingly, so they would have max cleaning w/ minimal part complexity.