Telling someone that is overweight that they are overweight, even in the guise of "looking out for their health", is like saying water is wet - in this weight obsessed culture we have, it's more than likely they're very aware of their size, and in this society that values skinny over fat, reminding them of their size can be very offensive. You do you, let them do them. Unless their actions interfere with you or someone else, you really have no grounds to "intervene" in their lives.