Due to requests, here is a tutorial that will teach you the basic idea of how SD (Super Deformed) mecha should be proportioned. Please forgive the crappy quality of this tutorial, first on the quality of the images. Second, on the quality of the writing.
First and foremost. This tutorial is a guide. By no means is the art of squishing and deforming characters set to one orthodox method. But what this tutorial will do is give you the basics.
So to get things started. The biggest thing about chibis is knowing your proportions.
I know it sounds weird to even bother thinking of proportions for something like that, but it's true even for chibi stuff.
For this tutorial, we'll be using Getter 2 as an example. Forgive the crude quality but I sketched these during class.
Basically the proportions for a chibi are the reverse of normal proportions. Essentially, the main proportions can be reduced to the Head, Body/Torso, and Legs. Take the rough size of these proportions and flip them around. Those are basic Super Deformed proportions. The length of space you'd give to the legs, give that to the head now. Stretch out the chest to compensate for the huge head. And then squish and stubby the legs. That's the core of classic chibi proportions. The classic proportions are most well known for the old SD Gundam illustrations (back when they had googly eyes).
From there it's just a matter of exaggerating key features to make them look amusing and/or cute. Like making the eyes ridiculously huge, or taking some key factor you identify the character with and just doing something weird with it like making it either super tiny for an "awww" effect, or ridiculously huge to bring attention to it. This is mostly learned through intuition and practice. There's no real set focus on what you should exaggerate exactly. Though generally you want to emphasize the eyes.
Now these proportions are just the basics. Most of the SDs I draw are using the recent Super Robot Wars proportions. These proportions are noticably different from the classic ones in that the character's legs are given more emphasis and the head size has been toned down a bit. I like to call this style of proportions, "Toddler Deformed" because it reminds me roughly of the proportions of a small child.
I hope this guide has shed some light on how you should be emphasising and proportioning parts when making SD's. But remember. Even with these proportions, the entire point of SD's and chibis is to be cute!! So long as it looks small, midgety, and goofy. Your SD/chibi has done its job.