Right click the scale on the left side of the windows and set it for db. I think the Wavelab 5 scale viewed from 0 db to about -50 db initially. The Wavelab 6 scale probably goes from 0 to minus infinity or -200 db initially, like Wavelab 9 does. I think that’s probably the main reason the waveforms would look different: they’re covering different level ranges, and the wider range will possibly look more compressed because it covers a much wider level range in the same vertical space.
Although there might also be additional differences in the scaling besides that, that seems the most likely thing to cause a difference to me.