Koko is nowhere near flat. If a woman can get a noticeable bulge while wearing a suit there’s definitely some material in there. And bad support, like I mentioned earlier.
And Valmet, despite being muscular as all hell, has always shown that she’s got big ones that aren’t hard-packed muscles like most female bodybuilders have. Off the top of my head, there’s that episode where Chinatsu and the other guy showed up when she was shopping with Koko. She was running from cover to Koko when she got shot in the leg and fell. There was boob bouncing and spilling everywhere.