In order to make a set of plans I was happy with, I had to make a few decisions about how to interpret parts of the original Zaku Kai design. Depending on your perspective some of these decisions could be considered assumptions, logical conclusions, or just simple matters of personal taste. To me, however, the results of these decisions are an essential part of structuring the design process. Without this level of analysis I can intuitively look at a drawing or model of the Zaku Kai and recognize that there may be something wrong, but not necessarily know how it should be fixed. By trying to define the specific traits I think should be in a Zaku Kai design, the design work becomes more structured and productive.
The Zaku Kai is generally regarded as a fairly bulky design, and to a certain extent this is true. However, I think that a common error in rendering the design is to attempt to improve the rendering by adding bulk. Any number of things could be wrong with the rendering, but people will still conclude that the problem is a lack of the Zaku Kai's characteristic bulk. My reaction to this is that people overestimate the Zaku Kai's bulk, and fail to recognize the specific design decisions that lend it the effect of having great mass.
Basically, the mass of the Zaku Kai is distributed much differently than other Zakus, but it is actually not much bigger. The impression of size is a result of the relative size of one area of the machine compared to another. The small head, for instance, makes the chest block look larger, while the large shoulder armor and skirt armor serve to keep the chest block's apparent bulk from looking out-of-place. Because of all this I feel like it's important to remember that the error in the design isn't always where you think it is. If you think a chest block is too narrow, it's also possible that it's too tall, or too short front-to-back, or that the parts surrounding it are too big.
The Zaku Kai's chest block is divided into three segments - the two outer blocks, and the center block which contains the cockpit and such. From my observations I decided that the three segments are of approximately the same width, so that the chest block as a whole is divided into three roughly equal parts. The width of the head is roughly equal to the width of the center area of the chest block.
While the chest area of the Zaku Kai is very blocky, almost every edge or surface is at some non-perpendicular angle to the horizontal or vertical. In a front view the chest block defines an upward sweep - the upper edges, lower edges, front edges, etc. of the chest block all create this upward-sweeping effect, which is continued with the shoulders, which rest very high relative to the chest and head. Likewise, the sides of all three sections of the chest block are not vertical: they are all angled outward.
The skirt armor is significantly wider than that on other Zakus: most Zakus' skirt armor wraps snugly around the top of the legs, but the Zaku Kai skirts shown in the lineart have room to spare. The bulk of this area helps to balance out the look of the chest block and continue the effect of bulk into the lower half of the machine. The upper and lower contours of the skirts also echo the upward sweep of the chest block.
One element of the design I found challenging to interpret was the edge on the back of the chest block, near the bottom of the backpack. In the lineart this line looks horizontal, but if the edge is horizontal then the trailing edge of the bottom surface of the chest block must sweep forward significantly near the center of the chest, in order for the two edges to be coplanar. I decided to reduce this effect by making the back faces of the chest block also extend farther back near the backpack - I think that's a good effect even without taking the horizontal edge problem into account.
As previously noted, the shoulders sit high and help define the curve across the upper and lower contours of the chest. The shoulder armor is nearly as tall as the chest block, helping to balance out the bulky look of the chest, and offering plenty of room for the left arm to move.
I found it interesting to try to determine the interaction between the shoulder itself and the upper section of the arm: the upper right arm terminates more or less at the rectangular shoulder block, and the upper left arm terminates somewhere inside the spiked shoulder armor. The upper arms also have little tabs on them, that appear to stick up higher into the shoulder. I felt there were different ways these features could be interpreted, but the way that appealed to me most was to make the shoulder joint spherical (taking a cue from various "Char's Counterattack" designs) and make this tab a part of the upper arm, which would slide over the surface of the sphere as the upper arm pivots or extends laterally. In the right shoulder, the shoulder box will sit on top of this sphere - the left shoulder, I've decided, will have no such shoulder box, as the spiked shoulder armor already protects the shoulder joint.
To me, one of the most striking features of the Zaku Kai's design is the legs. The legs are far bulkier than in previous Zakus - except, perhaps, for the Hi-Zack - and I think their general shape is quite appealing.
The lower legs are comprised of the relatively slender, and long (front-to-back) core segment, which is surrounded by the bulbous leg pods. The contour of the leg pods has always been difficult for me to visualize - after several iterations of my design plan for the leg I came to the conclusion that the leg pods are roughly egg-shaped, with most of the bulk of the pod toward the bottom end of the leg. The top and front edges of the pod are also distanced a bit from the top and front edges of the core part of the leg: a common mistake in rendering the Zaku Kai and F2 is to put those edges too close together, and lose the blank space there.
Perhaps the most distinguishing trait of the lower legs, to me, is the curve defined by the lower leg, flowing into the upper contour of the foot. Replicating this effect requires that the lower leg become fairly large near the bottom, and also requires a very large foot. Admittedly, from my experiments with the Blender model, a foot that large can cause problems: in standing poses it looks nice but in other poses the size of the foot can look overwhelming.
While the Zaku Kai is larger than other Zakus, a lot of the design is also specifically formed to emphasize that effect, too. The lower legs, skirts, and chest block all taper toward the top, as if to give a forced-perspective foreshortening effect. The robot's normal posture is like a "tough guy" pose, or a football player with pads on. Rather than overemphasizing actual bulk in my design, I want to use the effect of bulk, as well.