Photo screenshot file size in iOS 11.0.2

Hello,

Now I have problem with the file size of my photo which I take from my screenshots.

After I updated to iOS ver.11.0.2, I found that the Photo file size increased around 2 times.
(My device is iPhone 7 Plus)

(For example, Photo from my screenshot in the game, in the past the file size is around 3-6 MB, but after I updated to iOS 11.0.2 the file size increased to be 8-11 MB)(considered from Photo that take from the same game in my phone.)

This cause me troubles, because when I going to send my photo to some web-site or chat it showed that my file size was too big.
(I have to send Photo to my computer to reduced file size everytime....)

Apple Support in my country said this problem cannot solve now. (They said this is the performance of iOS 11 to make photo have high quality.)

And in Setting Menu, there is no method to reduce file size as well.

So, I would like to ask if anyone has any suggestion for me?

Thank you very much.