.BIN files are usually bigger than .ISO files, as .ISO files only store the files and folders, whereas a .BIN file is a true bit by bit copy.
.BIN files are images extracted in pure raw format (which is why they are sometimes renamed to .RAW files). That is 2352 bytes sectors, the full CD sector content: User data, sector header, error correction codes (ECC) and error detection codes (EDC). Once again, each sector is converted to digital data in the .BIN file, but more stuff than for an .ISO file is copied and the resulting file will be bigger. The .BIN file should usually be 251,000 x 2352 = 590,352,000 bytes in size. This process will copy ANYTHING on the disc, so it is useful for exotic discs (multiple tracks, mixed track type Audio+Data or Data+Audio) and for non-PC CDs (PSX, VCD, MAC).