iphone - Big difference on file size between iOS picture on the local and on server -

there image url http://example.com/xxoo.jpg

1. open url chrome, save image desktop check image file size.
2. open url iphone safari, save image camera roll, send computer check image file size.
3. save image in ios app using uiimagewritetosavedphotosalbum, , send computer check image file size.

strangely, these files different each other. difference can hundreds of kb.

i had tried download image using afnetworking setimagewithurl, sdimageview setimagewithurl, sdimagedownloader downloadimagewithurl , datawithcontentsofurl. these downloads same size different size on server.

there example code:

    [[sdwebimagedownloader shareddownloader] downloadimagewithurl:imageurl  options:0 progress:^(nsinteger receivedsize, nsinteger expectedsize) {      } completed:^(uiimage *image, nsdata *data, nserror *error, bool finished) {         _imagetobedownload = image;         uiimagewritetosavedphotosalbum(image, nil, nil, nil);     }]; 

these possible reasons:

• the camera roll may use different file format original file format on server.

• sdwebimagedownloaderreturns image not nsdataobject containing jpeg file server. when saving image uiimagewritetosavedphotosalbum file format can different on server.

• proxies in internet per default allowed convert media types (in particular images) different one, possibly higher compression level in order save space in cache. so, request may return cached version of original file on server has lower resolution. (reference: 14.9.5 no-transform directive, rfc 2616)