GIFs compress by removing horizontal redundancy. The more vertical detail or noise in GIF images, the less compressed the file will be. Dithering to "improve" colour is especially bad for compressing.
PNG behaves like GIF in that it compresses horizontal patterns, but it also
handles vertical patterns and dithering.
|GIF vs. PNG Compression|
|Gif Size||150 bytes||334||514||254|
|Format||Typical Compression Ratios||Description|
|GIF||4:1 - 10:1||Lossless for images (<=256 colors). Best suited for flat color, sharp-edged art. Horizontally oriented bands of colour compress better than vertically oriented bands of colour.|
|PNG||10-30% smaller than GIFs||PNG's similaiar to GIFs but compresses both horizontally and vertically bands of colour.|
|JFIF (JPEG)||10:1 - 20:1||High quality - has little or no loss in image quality with continuous tone originals. Poor for flat color, sharp-edge art, and bilevel images.|
|30:1 - 50:1||Moderate quality - generally the best level for the Web.|
|60:1 - 100:1||Poor quality - suitable for thumbnails and previews. Marked blockiness and Gibb's effect occur.|