Is undergearing ever bad?

[jocktheglide]

I have a tank savage now it weighs 17lbs or so and I was wondering can I go lower on the pinion due to the increase in weight so not much of a strain on the esc/motor? Its stock 20 pinion and i did run with 25 pinion that comes with it for 3 mins and the fan kicked on which is kinda early it seems packs were cool, motor, esc also to touch. I was thinking 18 tooth pinion to compensate for the weight.