I mean, actors doing voice overs still get paid millions of dollars for doing a Disney/Pixar movie don't they?
Not always. Typically, voices in animated films (even famous ones) are paid "scale" or the minimum wage for a SAG member. The studios typically pitch it to actors as a way to impress their kids, or as a way to boost their careers (see, for example, Ellen after her role in Finding Nemo).
For sequels, where the studios have less flexibility in casting, actors will often be paid the $millions figure you expect (I believe the precedent for this was Aladdin 2, for which Robin Williams was paid $10m).