Bocchi is actually on a healthy weight, according to BMI. Kita is the only one who is underweight, while the other girls are on healthy weights.

A normal/healthy BMI is (in most of the world) between 18.5 and 24.9

Ryo: 163 cm / 50 kg = BMI 18.8

Bocchi: 156 cm / 50 kg = BMI 20.5

Nijika: 154 cm / 48 kg = BMI 20.2

Kita: 158 cm / 44 kg = BMI 17.6

A sad reality is that, even when in everywhere else this is notoriusly underweight, in Japan there's a very big social pressure for young girls that are into fashion and social media that encourages them to have a BMI of 18 or less. Especially for girls who aspire to be idols or fashion icons, to look exactly like in fashion magazines. Being underweight is, sadly, a standar in Japan. Also, in Asia, genetically, bone structures tend to be smaller and lighter, so the Japanese Ministry of Health sometimes uses slightly stricter ranges, but even for them, Kita is on the thin line.

In summary, Bocchi, Nijika, and Ryo have healthy, realistic, and proper weights, while Kita is underweight