I would presume the iPad 5 would something around 500-525g, giving a leverage of 100g for the retina and 125g for the bigger screen, ( compared to the mini).
Reasons for such a drastic drop
New design, reduced dimensions
Thinner Bezels
GF/2 Display tech
single LED
IGZO should help reduce glass thickness
Thinner / Lighter Battery
Smaller sized SoC, as well as other internal parts.