maybe apple just worried for quad-core in 13" because :
1. overheat
2. high rpm fan rotation
... if we imagine in car, like BMW maybe :
13" is like 320/330i series
13" retina is like 325 sport series
15" is like 520/530/525i series
15" retina with high spec is like m6/750 series
all class is build for a market segmentation

, when u want high spec, you need upgrade your "size of car".