实现方案来自度娘

1.在SDWebImage目录下的 UIImage+MultiFormat.m 文件中增加方法

+ (UIImage *)compressImageWith:(UIImage *)image {
  float imageWidth = image.size.width;
  float imageHeight = image.size.height;
  float width = 640;
  float height = image.size.height/(image.size.width/width);
  float widthScale = imageWidth /width;
  float heightScale = imageHeight /height;
  // 创建一个bitmap的context
  // 并把它设置成为当前正在使用的context
  UIGraphicsBeginImageContext(CGSizeMake(width, height));
  if (widthScale > heightScale) {
    [image drawInRect:CGRectMake(0, 0, imageWidth /heightScale , height)];
   } else {
    [image drawInRect:CGRectMake(0, 0, width , imageHeight /widthScale)];
   }
  // 从当前context中创建一个改变大小后的图片
  UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
  // 使当前的context出堆栈
  UIGraphicsEndImageContext();
  return newImage;
}

2.在该文件下面方法中调用

+ (nullable UIImage *)sd_imageWithData:(nullable NSData *)data {
  if (!data) { return nil; }
  UIImage *image;
  SDImageFormat imageFormat = [NSData sd_imageFormatForImageData:data];
  if (imageFormat == SDImageFormatGIF) {
    image = [UIImage sd_animatedGIFWithData:data];
  }
#ifdef SD_WEBP
  else if (imageFormat == SDImageFormatWebP) {
    image = [UIImage sd_imageWithWebPData:data];
  }
#endif
  else {
  image = [[UIImage alloc] initWithData:data];
  //在这里增加下面三行代码
  if (data.length/1024 > 128) {
    image = [self compressImageWith:image];
  }
#if SD_UIKIT || SD_WATCH
  UIImageOrientation orientation = [self sd_imageOrientationFromImageData:data];
  if (orientation != UIImageOrientationUp) {
    image = [UIImage imageWithCGImage:image.CGImage
    scale:image.scale orientation:orientation];
  }
#endif
  }
  return image;
}