My iPhone has more than 100GB of system storage

Restore the iPhone.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

palmaga wrote:

Is there a way to reduce the size of the system without restoring?

Any other way would be more drastic; e.g., completely erasing the iPhone as if you were preparing to sell it.

palmaga wrote:

Also, could you help me by saying what are the things which run the system so heavy?

No, not without a detailed history of exactly how you treated the iPhone to this point.

palmaga wrote:

My iPhone has more than 100GB of system storage, moreover is taking over almost 50% of my internal capacity and even though is not slowing down the mobile, I would like to ask how can I clean it?

I wouldn't worry about it. Most of it is probably stuff that will be removed automatically if the space is needed such as caches.

If it really bothers you, then perhaps back up everything to iTunes, and then restore it from the backup. I've done that a few times with an iPhone 4s or iPad Mini 2. However, those had 16 GB total capacity and I was having noticeable issues such as not being able to update iOS without freeing up some capacity. Perhaps read this for pointers:

https://www.imore.com/how-find-and-remove-other-files-iphone-and-ipad

256 GB? I'm jealous.

sberman wrote:

palmaga wrote:

Is there a way to reduce the size of the system without restoring?

Any other way would be more drastic; e.g., completely erasing the iPhone as if you were preparing to sell it.

It's not necessarily that drastic. I've actually done that before. It does work when new Apps or an iOS update won't install, even though a lot of the "other" storage is going to data that theoretically should be erased.

The way to do it is to just go to iTunes and do "Back Up Now", followed by "Restore Backup". I remember recovering up to 1.5 GB on a 16 GB device that way. It did start going up again, but at least I could get it to perform an update. It might have affected things like my list of phone calls.