Shrinking components when the chosen smaller process has been just made available can be and remain expensive for some time and a large number of units produced, but doing it choosing an already mature smaller process can start saving money very soon. If power consumption is lower enough to allow using a smaller case, raw materials, warehousing and shipping costs decrease too, and on millions or tens million units it can give quite nice economies of scale. So the conditions are three: 1) choosing a shrunk process already mature; 2) such process must offer significant economies of scale and lower power consumption compared to the current one; 3) expecting to produce the new units in a number large enough.
A final benefit: in its last years, when PS3 will cost a lot less than now, retailers will appreciate that it occupy less shelf space, a Mini model helps extending the product's life.