Kiyoko's thighs and tits go up in size along with her tail count, which rises as she has kids prior to being released from the orb. She maxes out at 32 kits, and at that point I'd say she's got quite large thighs and ass, though her breasts are only large for a person in the real world. In Savarra, her maximum breast size is practically moderate.
Arona has large thighs and tits too, plus a wide frame, though I'd guess you don't consider her to be "thicc" because her bulk is from muscule rather than fat. Brienne also falls under this consideration. Azyrran, while using her alternative set to bimboify her body, still has clearly defined abs, so I'm assuming you'd categorize her as not being "real thicc" either. Etheryn can get transformed to have large tits, hips, and ass and she doesn't have abs, but I don't know how you'd categorize her either, as her waist is relatively thin.
Cait, meanwhile, is capable of reaching the by-far most plush body of all the companions. If she can't match what you want for a "real thicc girl", I don't think you'll ever get what you want added to the game in any capacity. If you just want big tits, you can give her a load of Bovum Sherry to give her frankly impractically large tits. If you want to increase her overall bulk, you need to have her do gangbang blowjobs with the guards at the gate leading into Khor'minos from the outskirts