The GPU resize could allow by this estimate a 35% increase in the number of dies per wafer, or say a 35% reduction in cost assuming similar yields/cost of process etc.
http://www.tomshardware.com/news/microsoft-xbox-game,5335.html
Then consider simpler power regulation and supply on the motherboard, so simpler trace lines mean fewer layers would be needed on the PCB and lower absolute cost. You also have a smaller powersupply unit required because the power requirements are smaller.
Theres also the consideration that a smaller GPU requires less elaborate cooling, they could use Aluminium instead of copper for example (Not sure which metal they use to be honest) Or just use less of it.
Also every product has an expected failure rate, the cost of replacement/repair is added to the cost of the product. If the failure rate is lower then this cost is obviously lower as well.
Tease.
