The 'poo' underwear, as well as the problem with the chest, turned out to be as much a problem of the uv0 itself, as it was a problem of the incorrect overlay of the sim, to which I aligned the uv0. By the way, regarding the alignment of loops during stretching (deformation), lighten and darken also work well, they allow you to remove (correct) unnecessary geometry.

Before the 1.5 times increase, I did not guess, I increased spine0 by eye. I tried this when I was aligning the sides and belly on the mesh. If there are 256 shades in the palette, what do saturation and brightness affect? It's tempting to make a big, square sim (something like building out of rectangular blocks) and color it in HSV. The result is depressing (the first run did not work, not everything is so simple). The ligaments were wild - biceps and stomach, lower arm and thigh, etc. Almost all the sliders for the body moved out. It happened by chance that I compressed half of the uv1 by x by 50% and packed it into the game in this form. Then what is the role of UV1? Again, it is not clear where the sliders are located. So, the weight map for l and r breast determines how the breast will grow (the smaller the influence of the group of vertices, the smaller the change). If I understand correctly, there is a direct relationship between the increase in a certain part of the body (buttocks, abdomen, hips, legs, chest, etc.) and certain groups of vertices (pelvis, spine0, spine1, tights, CAS_breast).

For the sake of interest, I aligned half of UV1, left the other half (just aligned the island, moving the vertices relative to the seams). If I understand correctly, this is a separate UV, which has nothing to do with uv0 at all. There may be some bugs that I didn't notice. I haven't made my own packs yet, just replaced the top and bottom mesh in Eve.

Sims 4 animated breast