hmm i've heard about Hanbin involvement in whistle. i didn't knew they gave him credits on the song after all.
YG did but after a while but it was a big deal back then because for a long time he didn't get credit for it.
anyways, my point is that it's obvious the girls are pretty involved in their music and just bc they don't get writing credits doesn't means they don't contribute in anything, if you watch some of the making of their music videos they talk a bit about things they came out with like concepts, ideas , etc.
so I'm pretty sure that the skinny bith part was lisa's idea it was something she had to deal with since was constantly bodyshamed for being skinny.