I recently noticed that when uploading images into Magento CMS > Pages > WYSIWYG it increases the image disk size. So if I am uploading an image with size 100 KB Magento makes it 300 KB after the successful upload. I think this is because of Gd2 when image is recreated, it gets bigger.
How to retain the original image quality and size when uploading an image?
I realized this recently too. I found this bug report from July, https://magento.com/tech-resources/bug-tracking/issue/index/id/1732/, that makes it look like it was introduced with SUPEE 9767. SUPEE 10266 introduced an option to bypass the new image reprocessing from 9767, but then you're defeating the purpose of that portion of the patch by turning it back off. There isn't an option to disable from the admin panel so it has to be done directly in the database. The reprocessing saves the image at 100% quality, and 100% is a static value so you can't control it without either bypassing, or overriding the core file. Or don't rely on the wysiwyg and instead upload images directly to your server via FTP or some other transfer protocol.
Here another one I came across which may be helpful: https://magento.stackexchange.com/questions/186405/supee-9767-patch-increasing-jpg-image-sizes
I didn't spend enough time looking into this. Directly FTPing images to media/wysiwyg/ folder was easy fix for me.