I don't see how the thickness of the belts would make any difference.Here's a test you can do for backlash. Move the X and Y axis around a bit using pronterface, so that the holding current is turned on. Then try to move it by hand. If you can easily wiggle the Y axis back and forth a little bit, then there is something wrong mechanically. Ideally, the printer should hold it firmly in place and you should not be able to move it until you apply a fairly significant force.