When the minecraft:scale attribute is modified, the entity shadow scales proportionally to the entity itself. However, this is not the case with slimes and magma cubes. Their entity shadow size scales only with the Size NBT tag. Adjusting the minecraft:scale attribute does not affect the shadow size in any way.
Phantoms, which also have the Size tag, appear to be working properly.
In my brief testing, the shadow scaling for slimes and magma cubes works fine in 1.21.3, but not 24w46a.

A magma cube so dense it bends light around it