Well it might be that Hinnuu being much more athletic might weight more due to muscle weight (Muscle is said to weight more than fat) and Arima strikes me as someone who doesn't have much muscle (but a decent amount of plushness chest and rear wise)
Bust...Waist...ButWhen it comes to weight...!?Ah yeah, I can see...Senne!!What!?How many grams are your tits-!!
Why is your weight less than mine...!These are Senne's physical examination results--!