Eating so much fish seems to have upgraded her chest quite a bit
Rather, it seems like half the light cruisers and destroyers have been secretly fed a fatty diet to suddenly make them way more busty in seasonal art. Just look at "Wait, Oboro has tits?!" or "Oh my god, Murasame, what happened to you?!"
If I didn't know better I'd say that originally, the artists were told to keep all the destroyers and most of the light cruisers flat, but after Ushio and the like became popular, they intensely regretted following along, so they're trying to countermand their previous works with seasonal art.
Must be a different case with BBs, specifically Yamashiro, because her sizable fuel tanks seem to have mysteriously disappeared.
In that case, hordes of fans assured her that she didn't need a Type-91 stuffed bra, and she grew more confidence in having "mere" large breasts. (Or they just would be too obvious in a bikini...)