The Rev. Al Sharpton, who was heavyset when he ran for president in 2004, said he has noticed both Ms. Gillibrand and Mr. Booker’s thinning profiles.

Recently, Mr. Sharpton, the civil rights leader who lost about 175 pounds, met privately with Ms. Gillibrand before a joint news conference. She marveled at his ability to keep the weight off. He returned the compliment, telling her she was “looking even sharper” of late, Mr. Sharpton recalled.

“And she says, ‘Well, reverend, you’ve got to always watch what you do.’ So I was saying, ‘Yes, especially if you have other plans.’ She said, ‘We’ll talk about that another time,’” Mr. Sharpton said of their exchange. “I was trying to bait her into telling me if she was going to run. But she did not give that away.”

All jokes aside, he said that a candidate’s weight can shape voter perception.

“Optics are important in politics,” Mr. Sharpton said. “And I think it does not hurt to look fit, because people want people that they feel take themselves seriously if they’re going to put the affairs of the state in their hands.”

In the past, Ms. Gillibrand has been open about her weight. In 2012 she detailed her hour-by-hour daily diet for Self magazine (“Snack: 10 almonds”), after losing 40 pounds after the birth of her second child.

She devoted a chapter about her body and gender expectations in her book, including sexist commentary congressional colleagues said to her (“You know, Kirsten, you’re even pretty when you’re fat” and “Don’t lose too much weight now. I like my girls chubby”).

“I don’t like being judged on my looks and, frankly, I’d like to spend less time thinking about my appearance, but there it is,” she wrote. But she came to see the benefit of opening up about her weight battles. “I’d always wanted voters to know that I’m a tenacious person, and what finally convinced them was that I’d possessed the determination to lose 50 pounds.”