The physics of moving the battery are more dramatic than they appear.
The front location is forward of the front axle, which means that the 70 lb. battery exerts more than 70 lbs. on the nose - by cantilevering, it removes weight from the rear axle.
The change to trunk mount is exactly the reverse: since the weight is behind the axle (and probably far more distant than the front location) it adds more than 70 lbs. to the rear axle weight since it removes weight from the nose.
The actual numbers can be had by measuring the distance from the center of the battery to the front axle, divided by the wheelbase, and multiplied by the battery (and tray) weight.
Example: if the battery center is 11" ahead of the axle and the wheelbase is 110", the ratio is 10%, so you get an extra 7 lbs. on the nose.
Do it the same way for the rear. The battery center is 20" behind the axle, the ratio is 18.2%, so the change includes:
1. the 70 lb. battery
2. the extra 7 lbs. on the nose
3. the extra 12.7 lbs. on the rear
totaling almost 90 lbs.

The rearward distance also helps prevent weight transfer slightly by increasing the polar moment - the leverage makes the tail end harder to rotate around the axle center.