First of all, make sure you have the imagick extension enabled within PHP. Just because ImageMagick is installed, does not mean that PHP is using it.
Before version 2.3.8 (7 months ago), Imsanity did not check the filesize of the new images during bulk resizing (it only checked during new uploads). It blindly resized the image, no matter what. So it isn’t a matter of anything being broken, it was fixed, and it can be difficult to understand why. The Bulk Resize Media plugin is based on a really old version of Imsanity, suffers the same flaw as old versions of Imsanity, and should not be used by anyone who values the security of their site.
Simply put, making the image dimension does NOT always make the filesize smaller. You can stop here if you don’t care why.
In my experience, using the imagick extension instead of GD usually produces smaller sizes, but again, it does NOT always make the filesize smaller. It does better, but it isn’t a perfect solution. It’s something core WordPress struggles with, when creating the various thumbnails/resizes as well. You might upload a 1500×1500 image, and when WordPress creates the “large” size, sometimes it is bigger than the original upload. It’s lower quality, smaller dimensions, but larger filesize. Sometimes even the “medium_large” and “medium” sizes will suffer from this same problem.
So, if you got this far, WHY? Why doesn’t a smaller set of dimensions automatically translate to smaller filesize?
An image is broken into small bits we call pixels. It is the smallest unit that can be displayed from an image. When you make the image smaller (dimensions), do the pixels get smaller? No, they can’t get any smaller. Instead, you have to throw some of them away, move them around, change the colors, and more to make the image look mostly the same as the larger one.
At this point, there is a general rule of compression you need to know. Smooth areas of images can be compressed better than rough areas that have lots of variation in color. For this reason, you might have 1,000 images taken by the same camera, that are all the same dimensions, and they will all be different in file size. And it all really comes down to the colors in the image, and how they are arranged. A picture of the pure blue sky will be really small, and a picture of a sunset will be larger. A picture of a bunch of flowers with different colors will be even larger than that, because there are more jagged lines, more variation, less room for compression.
Back to the resized image. This new image with smaller dimension will introduce more variation than the original. It will have more jagged edges, and it will not compress as well as the original. But there are less pixels to store in the file. So we have two competing forces here:
1. Less pixels generally means less data to store.
2. Less pixels of the same color with increased variation means each pixel requires more bits to store.
So when the resized image is saved, the end result depends upon which one wins. If the gains from #1 are larger, then your resized image is smaller, and you see sunshine and rainbows. If the loss from #2 is larger, then you are stuck with the original image.
So, as I said before, generally ImageMagick does a better job at making #1 win than GD, but not always.
The only other thing that can tip these scales within the context of WordPress is the compression level. If you make the JPG quality lower, you give #1 a better chance of winning at the cost of reduced image quality. If you raise it, #2 gets stronger, and the odds of getting a resized image decrease.
Hope that all makes sense, and please do NOT use Bulk Resize Media on your site ever again.
