Something about Anthony Jr.'s life and death inside ma had changed her in some unfixable way. She'd grown herself a big rear end and developed an unpredictable facial twitch. (2.9)
Ma's grief over her miscarriage manifests itself physically. This must be something she passes on to Dolores, who also gains weight when she goes through trauma of her own.
In January the hospital gave us back a new version of Ma: a smiling, twitchy woman with plucked eyebrows. (4.1)
Okay, so as we see from the previous quote, the twitch isn't new, but the eyebrows and the smile are. Ma has changed in a way that is similar to, but less dramatic than, the way Dolores will change after her own stint in a mental hospital later on.
"Five-five and a half," [the doctor] said. "Two hundred… fifty-seven." (8.67)
Here is the moment where we first realize just how much Dolores has transformed physically: She more than doubles in size after her rape as a result of binge eating to cope with the trauma.
Then I was dancing! They insisted. […] I felt free—a weightless astronaut, Carol Burnett without her fat suit. My long, gorgeous hair rocked from side to side. (11.196)
Dolores feels like a different person because she's acting like a different person: carefree and happy. She's not just weighted down by literal weight, she's weighted down by emotions, and letting them go helps her feel lighter, if only for a bit.
"Not with that hippopotamus!" (13.29)
For Kippy, when she sees Dolores, her roommate has transformed from someone like herself into a grotesque monster. For Dolores, Kippy transforms from a decent human being into a total jerk.
I was allowed to become [Kippy's] loyal, devoted servant. […] By the second week, she'd started calling me Dee instead of Dolores. (14.1)
Dolores only tends to get a nickname from people who are trying to manipulate her. Jack calls her Dolores del Rio, transforming her into a sex object, and Kippy calls her Dee, probably because she's too lazy to sound out her new servant's entire name.
As I began to drop weight, I began, as well, to drop my cattle prod of hostility whenever Dr. Shaw closed his office door. (17.36)
Dolores often hides behind her fat-girl persona, so when she drops the fat, she has to drop the attitude.
"You're rushing me," I said. "I'm only fifteen years old in the pool. How many kids my age have to work full-time?" (19.14)
Dolores hasn't worked at all by this point. She's in her early twenties, pretending to be a teenager, and still hasn't transformed into a responsible adult. Both twenty-something Dolores and fifteen-year-old Dolores are both resisting work—the transformative kind—here.
"You're a whole different person now, reparented and everything," I reminded myself. "Dante's waiting for you. Not that dead whale." (20.17)
The therapy may sound silly, but it really does change Dolores's way of thinking, and by changing her thinking, she's able to change her attitude.
He wasn't the boy in the letters. He was. Wasn't. Was. (20.234)
Dolores isn't the only one who has changed. Dante has transformed from his college days, too. For him, however, he lost a lot of his purity and replaced it with selfishness.