Take a look at your current (amps). It's far less on 6s than 2s. Higher amps = higher heat = less efficiency.

I have a 5th scale I converted to brushless and I run a 14s setup to keep my current under 80A and everything else runs cool. I could run it off of 6s w/ a higher kv motor but then I'd be frying my esc.

Good data, thanks for posting.