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The new iPad will most likely be a little less heavy knowing Apple will most definitely not create a heavier iPad. That is one thing that would hinder sales a bit IMO.
 
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.
 
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