If you have both trucks setup the same and the lighter truck is getting a motor running hot, but the esc is running at a decent temp, the lighter truck may be under geared. From the circumstances that you are describing, a hot motor would indicate undergearing. Try a pinion a couple of teeth larger and see what happens.

I'm sure that I'll catch some flaming for this, but ...you say that they are geared almost the same. I know that it looks that way on paper, but in practice, the setup with the smaller pinion is going to have less load on it than the motor with the larger pinion. I think that the stock pede is way undergeared. Try putting your 15T pinion on the stock pede and see what happens.