All dark colors make objects look smaller. This is true of furniture, fabric on a bed, house paint, and everything else that you can see. The reasons behind this have a lot to do with the way the eye works and how it perceives things that it sees. When you look at something, you tend to see the edges of it and to perceive those edges as the border of the object. This can be blurred when the object is dark because it appears to have fewer borders. A lighter color object will have lines, shapes, and perhaps partitions on it that are very visible to the eye. But when the object is very dark in color, none of those will be very visible at all.
This same principle works with clothing. For instance, if you are wearing a black T-shirt, you will look slimmer on top that if you are wearing a white one. If you have a very light T-shirt on, your shape will be noticeable within the fabric. Any curves, bulges, or flat areas will be noticeable beneath the white fabric. If the T-shirt is black however, that will not be the case. All that will be easily noticeable is that there is a field of black. This field will not have bulges or areas that look large unless the T-shirt is very tight. That creates a field that is slimmer and smaller looking than it would be otherwise.
Many people wear black often because it has this effect on the eye. If you are feeling self-conscious about having a large chest, having a less than flat stomach or any other figure problems on top, a black T-shirt will work wonders works on that area. The plain black field makes the bulges fade away to the eye and can and instill self-confidence for that reason.
Like a black T-shirt, a pair of black pants can be very slimming to the legs. If you have a lower stomach that is giving you trouble, or you have saddlebags or thick thighs, wearing a pair of black pants has the same effect that wearing a black T-shirt has on upper figure faults. It creates that same black field of space in which specific areas of the figure are not noticeable.
The way that light plays against dark and light colors is also a factor in the way that different colors make you look. Black does not reflect back the light that shines on it. A light color reflects light away from it. Both of these effects have an impact on how large the color makes an object book. Because black does not reflect the way the light, it is a deeper field color that makes itself look smaller. Because white does reflect light, its light color draws in the eye. If you don’t want the eye brought to your problem areas, make sure that you are wearing black. This will not only make you look leaner, it will give you the self-confidence you need to feel better about your figure.