Mutt good question, I did an iteration with comparable aircraft and technology involved.
It think what can make the Greenliner lighter then the similar sized widebody B767-200 and A310-300 are:
- full composite CFRP fuselage (757 fuselage= 727: sixties), wings and tail
- the absense of a serious cargo deck & required loading system/door and overall fuselage / LDG strenght for lower MTOW.
- new technology engines incorporating carbon reinforced fanblades & cowlings
- new technology like FBW, less pneumatics, cabling
- new lighter materials in cabins, galleys and seats
Still 10 tons heavier then the 757-300 because itīs longer, has bigger wings and larger payload and range.
As can be seen the Greenliner isnīt extremely light but the 787-3 seems relatively heavy & has low payload for the range it offers. I doubt the 787-3 will be a succes, the 787-8 offers so much more flexibility for an additional 10 % OEW..