In the provided example code I see that you cancel the OnImageSaving event. At the same time there is only code that generates 2 thumbnails, but does not save the main image. Since you cancel the event, it should expected that the image saving to be handled in the event handler, because the ImageEditor itself will not save the image. This may be the reason why the image has larger size than expected, i.e. the original size.
You have 2 options here - one is to not cancel the event and let the ImageEditor store the changes in the image you are editing, the second option is to handle the saving of the image in the OnImageSaving event handler.
Hope this helps.
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