Empty space
Leaving empty space in your bot is a common practice among newbies (See also: N00b Mistake).
But, it is a bad idea. Every real life builder will agree with it, and all the good RA2 builders will tell you "don't let empty space in your bot", simply because the more armor plates it has, the heavier it is. However, when the added few kilograms that can be gained from using a perfect chassis can not be used for anything really beneficial, it is better to build the chassis so you go all the way up to the weight limit. This is because the larger the chassis is, the more HP it has (credits to Clickbeetle for figuring that one out). For example, a middleweight with a chassis that weighs 395.1 with a perfect chassis is worse than the same MW with a weight of 398.9.
Empty space can waste as much as 100 kg. So, pack the components tightly into the chassis.
Please.
A good tight chassis is cooler than a chassis where you could pack 3 elephants.