I love Susan Strong's design, although I wish she wasn't just another Aryan blonde blue eyed human. I would have liked her more as a woman of color, just for a bit of racial diversity in the show. Maybe the choice was a conscious one, to make her eventually turn out to be related to Finn, but... ehn. I was pleasantly surprised by her amazonian bulkiness. I found her to be overall a very charming character.
I read LSP's "lumpyness" as coding for being fat, which is one of the reasons I adore the character so much. She's lumpy and she considers it the hottest thing about her. She FLAUNTS DOSE LUMPS. You go, LSP.
There's been some debates on Tumblr whether Fionna is "chubby" or not, but in my headcanon, she is. I love the idea of a curvy action hero girl.
PB is a bit harder for me to categorize. When I imagine her as more realistically rendered, I don't think of her as being stick thin as she is in this heavily stylized style. I think she'd be less curvy than Fionna, and taller.
If I had included Marceline in the lineup, I think she'd be the mostly lanky and hunched over. Marceline in my mind is rather androgynously proportioned, almost gangly.