Ram speed for the core series is negligible you'll see minimal gains because ram throughput isn't just about speed but a combination of speed and timing. Higher speed RAM often has higher latency and as a result performs the same as slower speed RAM. It has more to do with the manufacturer of the RAM itself rather than one being faster than another. Think of it more along the lines of tech vs horsepower. Just because one is a pushrod,flat,v-config,iron block doesn't matter per say when they all put out the same horsepower. Its just about how you get there, for example
1-1-1-10 @ 100mhz
2-2-2-10 @ 200mhz
Are essentially the same in terms of throughput so:
10-11-10-30 @ 1866
9-9-9-24 @ 1600
= about 10% difference in latency so the 1600 is actually ~1760mhz effective so the 1866 is faster but not by much. See?
TLDR: The difference you're seeing is because the 15" use the 4th gen Crystalwell and the 13" uses 5th Broadwell. They are both roughly about the same the only difference is that BW is a die shrink of CW to save power so the RAM is different as a result.
