Even if you looked in the same mirror you will look different if the lighting conditions changed. If the light fell on a part of your body and created a shade then you might appear fatter than usual.
According to experts a slight curvature along one axis can make the person seem either thin or fat depending on the nature of the curve. Because some mirrors are manufactured differently they can make you look fatter. See why do i look different in different mirrors.
Thin mirrors are more likely to get bent. If the mirror's glass is bent then the reflection of your picture will change and so you might appear slimmer or fatter. Thick mirrors are thus more likely to give more accurate pictures. See why some mirrors can make you ugly.
Most Mirrors bend over time. As the mirror gets older it might bend and thus reflect light in a different way. This change can make you look different.
Some mirrors can make you seem shorter if they were bent in a certain way. This is more likely to happen with old mirrors that bend under the effect of their own weight. If you appeared shorter then you might assume that you look fat.
According to psychologists your thoughts and concerns can affect your perception and let you see something just because you are concerned about it. If you are concerned about your weight then you might see yourself fatter.
If the strong light source is falling directly above your head then probably your body is going to appear lumpy. In such a case the shadows created by that light can make you seem fatter.
If the walls of the room absorb light because they have a certain color that does so then this might create a bad reflection that could make you look unattractive, old and fat. See why do i look bad in photos.
If the light in the room is very bright then it might show the imperfections in the body thus make you seem fatter. See also why do i look ugly in sunlight.
White light can be harsh when it comes to showing imperfections. It can thus make a person seem fatter. See why you look different in different lightening.