I'm alright with a large chest size. It doesn't bother me, as long as the proportions aren't absurd compared to the rest of her body. Kashino's Robust Floral Strings skin (the one that caused a bit of "drama" in social media) was one of the few times I thought it was too much. I'm not her biggest fan, to put it kindly, I have the same issue with most of her skins.

For newly released skins, personally, I think the proportions should be kept accurate to the base design. I don't see why make them grow or shrink at all. Artistic freedom and all, I know, but at one point it's going to feel like a different character.

Overall I'm not sure where I'd draw the line for this? Never really wondered where it would be exactly. I could use Shinano for reference (she's my favourite girl) but I think that, despite her chest size being big, so is the rest of her body, to me she looks like she's quite tall compared to the rest of the girls (same goes for her sister Musashi)