Raster graphics, also called bitmap graphics, are digital images that are composed of tiny rectangular pixels, or picture elements, that are arranged in a grid or raster of x and y coordinates (includes a z coordinate in case of 3D) in such a way that it forms an image. It is also referred to as bitmap since it has information that is mapped directly to the display’s grid.
Raster graphics can typically be scaled down with no loss of quality, but enlarging a bitmap image causes it to look blocky and "pixelated." For this reason, vector graphics are often used for certain images, such as company logos, which need to be scaled to different sizes.
The file size of a raster image depends also on the size of the image, which is determined by the number of pixels being used in the image. This means that an image with a 1280x720 resolution will contain 921,600 pixels while a full HD 1920x1080 image will have 2,073,600 pixels, which will obviously give it a bigger file size when compared to the former.