I also can't understand this. I'm more familiar with the other direction, but even then, being underweight will lower testosterone way more than being overweight and raise estrogen by the same amount with the right diet. It also causes a lovely 0.7 waist to hip ratio rather than a 0.7 hip to waist ratio, which means fitting into actually cute clothes way sooner. And being thin makes the initial surge of weight gain when starting estrogen go right to your boobs and butt rather than anywhere else, makes you normal sized, and you don't have to deal with the very slow process of redistribution. It's wonderful.