The size of EPS (Encapsulated PostScript) format files can be rather large, as such files contain the image itself as well as special elements needed to install PostScript language printers. This language was developed by Adobe company to describe files which are being printed. If a printer supports this language, you can specify file size, used fonts and some other information when printing the image. This makes your printer "understand" what exactly it should print.
Files of EPS format contain descriptive information in their headers. Hence, such images can be embedded into text documents and documents of other types. Files of this format can be easily transferred from one computer to another, but they are too large and it takes too much time to process them, as graphics editors have to process the image as well as its PostScript instruction. It is evident, that these files are not suitable for Web publishing.