after complete backup and restore ios system size shows 20 gb

I recently have had a problem with the size of the System file being to large. I did a complete backup and

restore of my iPhone 7. I am updated to ios 11.2.6. After reinstalling the backup to the iPhone I saw that

the system size drop down to about 15 gb. from a 32 gb. iPhone 7. However after about a few hours I looked

at my system file size and it now says it is back at 20 gb. Can anyone explain what could be happening to

my iPhone 7.