The first two points are, but not really the RAM. It's going to be an ounce at most. If they go from two 8GB sticks to two 16GB sticks, it wouldn't change size/weight at all.
The reason Apple gave is the truth, LPDDR3 only supports 32GB. You would need DDR4 for 64GB but Intel only supports regular DDR4 not LPDDR4. While DDR4 uses less power when active, LP saves significant amounts of power during standby and they would need bigger batteries to compensate.
>- Thiner/Lighter
>- Longer Battery Life (5+ hours)
>- 32/64GB RAM
These goals are diametrically opposed.