plants like most living things are made of mostly water so when you dry you loose most your weight

there are few reasons it could have been loose/airy
light source too far away
light source to weak to penetrate canopy. what type of light do you have?
stress and strain

you can over dry buds, generally if you can break a branch and hear it snap, their generally about ready to put in jars, usually takes atleast 7-10 days before you jar up