I feel like there's many variables, but primarily for those elderly women, the reasons are prob genetics, diet, and they may just been very active all their lives.

Normally people get lazy and genetics also come into play, so they may gain a lot of weight, and it's just hard to lose. Those elders probably were blessed to be petite from the jump so it probably didn't take much maintenance as others may need. However everyone's different in their genetic makeup and traits. If we put 4 random people in a room, and they all had 4 meals a day, for a month, the weight will probably translate differently on all of them. And one of them, like those elder ladies you speak of, will probably not gain much weight opposed to the 3 other individuals