First questions is did you create the meshes you are importing? If not can you guarantee they were all saved at the same scale?
Scaling can be a major issue. Take this PG https://www.babylonjs-playground.com/#QY1WYT#0
and look at line 28, the rabbit has been reduced in scale to give it an appropriate size relative to the dude. Comment out line 28 and see what happens.
Changes in scale will also affect relative positioning.
You may want to read What is the standard length unit in a scene