We are caching images downloaded from our server. We write them to our local storage like this:

 NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
 NSString *documentsDirectory = [paths objectAtIndex:0] ; 
 NSString* folder = [[documentsDirectory stringByAppendingPathComponent:@"flook.images"] retain];
        NSString* fileName = [folder stringByAppendingFormat:@"/%@", aBaseFilename];

 BOOL writeSuccess = [anImageData writeToFile:fileName atomically:NO];

The downloaded images are always the expected size, around 45-85KB.

Later, we read images from our cache like this:

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
 NSString *documentsDirectory = [paths objectAtIndex:0] ; 
 NSString* folder = [[documentsDirectory stringByAppendingPathComponent:@"flook.images"] retain];
        NSString* fileName = [folder stringByAppendingFormat:@"/%@", aBaseFilename];

 image = [UIImage imageWithContentsOfFile:fileName];

Occasionally, the images returned from this cache read are much smaller because they are much more compressed - around 5-10KB. Has the OS done this to us?