As you’ll note in the first painting, the outfit is built to extend the shoulders out wider. The rest of the outfit tends to do this as well.
There are two possible reasons for this:
The clothing was designed to make people look wider, with the idea that wider looks cool/powerful/muscular/etc.
Traditional Japanese clothing always works on rectangular units. In Western clothes-making, any extra cloth beyond that needed to cover the body was considered waste. It was better to be trimmed and made into rags, patches, or whatever else – something useful. In Japan, if you had a rectangular bolt of cloth, you could fold it however you wanted, but you didn’t cut any off. This makes the clothing tend towards baggy.
My personal guess would be that the desire to make look people buff led to a preference for clothing that was baggy, which led people to realize that they could accomplish this by not trimming cloth to fit. Once this was all in place, the rather flamboyant aspect of the clothing would then get picked up and emphasized by the artists. Wanting to make the most powerful people look the most powerful, it would then have been found a rather simple shortcut to take the natural effect imparted by the clothes of largeness and extend it to the art – making the most important people the largest.
That’s not strictly speaking true. All kimono is made from cut bits of fabric, and some outfits, especially court outfits, require quite a bit of shaping. Yes, the basic shapes are overwhelmingly rectangles, but they were all cut panels. It’s just that the traditional Japanese fabric width is a lot less than the Western norm.
The answer is twofold:
wide was very much in fashion for formal portraits. Compare Western fashion of the time, where the beer belly was enshrined in the peascod doublet.
it was an artistic tradition for *formal *portraiture. One that stretched back to Chineseforbears, in fact. Combine that style with the later Japanese lack of chairs, and you get human pyramids. It’s easy enough to compare it with more action-oriented or informal contemporaneous stuff.
It’s not about clothing; it’s about art. That was the style that the subjects liked (they paid for it, so they got what they wanted) and they liked it because that’s what they were used to seeing.
It probably dates back to some painter who did it once and had his subject say, “Wow! That looks kewl!” (or the Japanese equivalent) and whose friends thought the same, thus commissioning more from that painter and from others who painted that way.
The rough equivalent currently is the “buff body” look, which as a fashion, pretty much dates back to Neal Adams’s work in comics.