Raster graphics are computer graphics that consist of a grid of pixels, also known as a bitmap. Most images on the Internet and your computer are raster graphics. They are one of the two primary image types for computer graphics with vector graphics.
The file size of a raster graphic depends on several variables. The first is a raster graphic's image size, also known as its resolution — the number of pixels it is wide by the number of pixels it is tall. A higher-resolution image has more pixels and thus requires more disk space to store. For example, an image 640 pixels wide by 480 pixels tall consists of 307,200 pixels total, while a larger 3072 x 2048 image requires 6,291,456 pixels. A raster graphic's file size also depends on its color depth — how many bits of data each pixel requires. An image with 8 bits of color depth can only display 256 colors. An image with 24 bits of color depth uses three times as much data but can use more than 16 million colors.
Since raster graphics consist of a grid of pixels, resizing one to change its resolution will necessarily change the contents of those pixels. Shrinking one down requires the data from multiple pixels to be combined, causing fine details to be lost. Scaling one up requires creating new data between the existing pixels, which can result in an image that's either blurry or blocky.
NOTE: There are many file formats used for raster graphics, some of which include image compression to reduce file size. For example, a large bitmap image saved in the lossy JPEG format will have a smaller file size than the same bitmap image saved as a losslessly-compressed PNG file, which in turn will be smaller than that image saved as an uncompressed BMP file.