iphone shrinks images on camera roll while cropping

Hi, Im a professional photographer; I need sometimes to edit on the go on my iphone 7 plus; I import jpgs images from my dsrl camera into the iphone with a cable, then edit them in Camera Roll and upload them to dropbox. My problem is, when I crop images, even by the slightest of margins, the images saved by the phone are compressed to a more powerful compression setting that I would like; so instead of having a file that normally would be 6 to 7 MB when slightly cropped in the powerbook, I get a file of lets say 1.5 MB when cropped on Camera Roll.

I don't want any other apps, or programs or e mailing the images to myself or sending to my computer etc,(no time!) I just need to be able to crop in Camera Roll using the highest compression setting as not to shrink the image more that it is necessary.