So your student is becoming overweight: does this negatively impact your research group in some way? Should an advisor care?
Becoming overweight is not an issue per se. Major physical changes, however, are often a sign of an major ongoing mental or physical health event. Rapid weight gain, for example, is often associated with depression or thyroid problems, both of which can have a major impact on the student's ability to learn and work. As such, I would definitely be concerned if a student was visibly gaining a large amount of weight---not because of attractiveness or disapproval of weight, but because of what else it might indicate.
I would find it highly inappropriate, however, to bring this up with the student by saying something like "I've noticed you've gained a lot of weight" because there are also a lot of more benign reasons somebody might gain weight, including recovering from mental or physical health problems that would also be none of my business.
I would, however, notice it as a possible red flag, and start to keep a more careful eye out for other signs of distress, which might legitimately trigger a conversation about, for example, mental health.
The supervisor should supervise students about their research work and not judge students based on their appearance or their private life. It would not have any impact on my research group.
It would help to know what motivates the question, to make sure I'm getting at what you are trying to find out, but here goes:
The advisors I have known would wait for the student to bring up a health situation before talking about it.
They would consider the overweight in conjunction with other aspects of the student's well-being. Being overweight by itself wouldn't be a concern. If it were accompanied by symptoms of pre-diabetes, heart disease, OCD, depression, etc., then the advisor would be concerned about the big picture (including, but not limited to, the weight problem).
The concern would be about the student, not about the group.
The gender of the student would be irrelevant.
This is my assessment, regarding the advisors I have personally known.