Does size matter?

While waiting for the green balls to be available I've introduced Nancy to a smaller silver ball (the ones that you can use for keep fit purposes).  She will sniff it, touch it with her foot, be happy with me rolling it around her and bouncing it and also placing it on her - I've just started to bounce it gently on her, BUT she will not attempt to push it.  Both Billie and Hatti push it around, but this is a definate no no for Nancy - she can't see the point.  Any suggestions?