>majority of planets that don't get air/space bombardment support will be planets with thick atmospheres or so hot all munitions will vaporise/detonate before reaching ground level
>the only planets that will not have colony support to mine out are high gravity/pressure planets
Planets like jupiter, uranus and venus. Which in most solar systems, are very common. More so then earths, mars and moons like titan.
This suggests that people who come into these environments will undergo large scale changes to their bodies to take the harsh conditions. Such as becoming large humanoid robot giants with lengthly changed composition to their brains. (a skeleton in almost any human is a dozen years old or less, it is replaced that often!)
This is the setting in which big mecha has the spotlight. Mining worlds such as venus, uranus and jupiter with modified brains of humans replaced materials with more durable compositions.
http://www.astronomytoday.com/astronomy/venus.html