Perhaps your idea about the weight of the upper pieces of the robot being greater than Niryo’s printed parts would explain why the arm drops so easily. As Edouard suggested in his reply above, maybe it’s not a critical issue so long as your robot is calibrating correctly and other issues aren’t occurring during normal movement.
Did you weigh the Arm Bottoms that you have printed (both 20% and 40% infill) and compare them to the weight I reported? Perhaps that might give you an idea of the approximate potential weight difference of the parts above. I could list out the weights of the rest of the parts in my kit if that would be helpful.