I'm sure she just runs the tub and throws in the bath bomb as a combination tub-cleaner/air freshener.You can roughly estimate how much water a human displaces by converting their weight to kilograms and assuming 1 kg displaces one L of water.
If Chantal weighs 400 lbs (we know she weighs more but just saying), she weighs 181 kg, which will displace 181 L, or roughly 58 gallons of water. If the average bath contains 45 gallons, Chantal will displace ALL the water in the bath.
But that’s assuming she tried to fit her entire body into it, laying down and everything with nothing sticking up above the rim of the bath (an impossible feat for Chantal and her giant ball-shaped body). If we assume she lowers herself into the bath (by some hydraulic miracle) and only the lower half of her body is submerged (legs, arse and fupa), we can generously say only slightly more than half her weight is displacing the water.
Meaning if Chantal tries to sit in a bath, she will displace approx 27 gallons of water, leaving 18 gallons in the bath with her. Not an awful lot.
However, if she weighs 450 lbs, as I strongly suspect, then applying the same parameters, she would sit down in the bath and displace all the water except for 9 gallons.
But I strongly suspect she’s incapable of actually fitting into the narrow bath in the first place. She may just sit on the side with her feet in it.