I'm thinking maybe your spine/discs compress during the day? You could try incorporating Pilates and/or yoga into your w/o routine. It helps strengthen and lengthen the spine. Look into it, might help keep things on an even keel.
__________________ Why didn't Noah swat those two mosquitoes?
The variations in height between morning and evening has to do with the vertebral discs. In the morning, as you have been resting all night, the discs are not so flat as in the evening, so you may have a difference of 1, 2 cms. As far as I know, heavy weight training does not affect height.