No, she does not need to be overweight for that to happen. My grandmother had the same problem. The room with the toilet simply was so small. There was exactly enough space between the toilet and the wall for her to fit in. And since she is very frail she could not get out herself anymore. In the end those incidents where she fell down and could not get up herself anymore became more and more. She quite often broke some bones when it happened and every now and then she couldn't even reach her alarm-button and had to spend a few hours on the cold stone floor until somebody found her. My aunt with home she was living in the same building finally got so paranoid that she could not sleep anymore because she was afraid that my grandmother would fall down again during one of there many nightly excursions to the toilet. So my grandmother now lives in a nice retirement home where they not only have heated floors and other stuff for her to be more comfortable if she falls down, but also night-nurses that check on her once an hour even without her pressing the alarm-button.