Noobish ESC load question...

So my great idea was to run my mostly aluminum/Ti 3905 w/ 1515 1Y and a Quark 125B on 5 cells for me and 3 cells to slow it down for my son. Stock spur and 21pinion, stock diffs (not for long) and 40 series mulchers.

Well it thermals after about 5 min on 3 cells (MaxAmps 5000x2 in Parallel)- the only batteries I have tried with the quark.

So does the ESC work harder (pull more amps) when trying to run a big motor on lower volts? i.e. it will run hotter on 3S than 5S under the same conditions? Or is the Quark just that temperamental (I have since put the Monster heat sink and 2 fans on it - but haven't run it)?

I ran 5S (2S+3S MaxAmps in series)on my new Mamba Monster I put in it and the fan only kicked on a couple times during a 20 minute bash. Didn't try 3S yet.

It is probably obvious what is happening, but I couldn't find the info searching... thanks for any help... I doubt a lot of people on this forum have even thought of running 3S in their trucks/buggies...