I suppose it depends, among other, on what they need the characters to do. In LBT 2 we see Petrie is able to only soften Ducky's fall when he graps her, in LBT 4 we see him comfortably carrying her several meters upwards. I suppose it is easier for the artists not to draw any of the dinosaurs too small while in certain scenes they have to appear smaller than usually (e.g. when a threatening effect needs to be shown by the sheer size of whoever is threatening, or when Ducky needs to take a seat on Littlefoot's head etc.).