Actually, it does change your weight based in your height, it's just that it calculates BMI first and doesn't factor in your height until later:
Code:
// Simple BMI: 20 is underweight, 25 is average, 30 is overweight
var nBMI:Number = (25 + ((thickness - 50) / 10));
if (hasPerk("Hollow Bones")) nBMI -= perkv1("Hollow Bones"); // usually 10
if (nBMI < 5) nBMI = 5; // to avoid total weirdshit
// Raw body weight, assume without extra parts
var nWeight: Number = ((tallness / 12) * nBMI * (tallness / 75));
If you want all the details, the full code (assuming you can understand it) is at
https://github.com/OXOIndustries/TiTS-Public/blob/master/classes/Creature.as, lines 21,487 to 21,730.
Also, just throwing it out there: my Steele is 2 metres (6'7") tall and weighs almost 92 kilograms (202.7 lbs) with full milk and cum, giving her a BMI of 23; without milk and cum, she's 83.4 kg (183.9 lbs), giving a BMI of 20.85. She has a Thickness of 25 and a Tone of 75. Of that mass, 3.6 kg (8 lbs) are in her breasts, 0.65 kg (1.4 lbs) are in her balls, 1.9 kg (4.2 lbs) are in her cock, and 5.4 kg (11.85 lbs) are in her butt.
Back when I wanted her to be rather chunky, she had a Thickness of 100 and a Tone of 0. This gave her a weight of 136.6 kg (301.2 lbs; BMI 34.15) with milk and cum, 109.4 kg (241.2 lbs; BMI 27.35) without.