1 Different light settings
Even if you look in the same mirror, you will look different if the lighting conditions change. If the light falls on a part of your body and creates a shade then you might appear fatter than usual.
2 Different mirror curves
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.
3 The mirror is thin
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.
4 The mirror is old
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.
5 Some mirrors make you shorter
Some mirrors can make you seem shorter if they are 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 appear shorter then you might assume that you look fat.
6 It's a perception problem
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.
7 Light is directly above your head
If the strong light source is falling directly above your head then your body is probably going to appear lumpy. In such a case, the shadows created by that light can make you seem fatter.
8 The walls absorb light
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.
9 The light is very bright
If the light in the room is very bright then it might show the imperfections in the body; thus making you seem fatter. See also why do I look ugly in sunlight.
10 The room has white light
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.