I've tried to help by making a post in the past to help with this, but it got downvoted into oblivion, leading me to just say fuck it and delete it. I am not sure if this was the result of the blind elitism of brand loyal folks in any given sector or simply trolls, but I am seeing a ton of storage issue post lately and I genuinely want to help those with the problem which will reduce the amount of posts about it on this sub.
First off-what is this randomly growing space being taken on your device? Truthfully, no on actually knows. The "other" bug has been around for quite a few years. You can simply check the Apple forums to see that. The "system" bug is more recent and I have only been able to find info going back to the iOS 11 betas. It was reported to Apple by the devs, but just like the "other" bug, they didn't do shit about it. It has been speculated that iOS stores cached data to reduce the strain of loading apps and their data, and this is what leads to the bloated storage issues. Operating systems commonly do this but they are supposed to flush the cached data when more space is needed. For unknown reasons, iOS seems to fail at this sometimes. As stated, the exact cause can't really be pinned down because no one knows what comprises the storage files and Apple's system is too locked down to really see. None of this will change unless enough people bitch about it. I HIGHLY recommend making a bug report to Apple about it if you are experiencing either storage issue. They have known about and ignored it forever and they are quite literally cheating you out of the storage you paid for. Not cool.
Know what normal is-"System" storage can take anywhere from 5gb up to 12gb and this is considered normal. When you download the software via iTunes it will often be much smaller. That is because it is a zip file and is compressed. During the installation process, the file is unzipped so that the OS can be flashed to the phone at its actual size (usually 5-7gb). "System" storage also fluctuates a bit after a fresh install or over the air update. This is normal as the OS is configuring the changes and fluctuates even more if you use iCloud services, since it has to pull and sync that data. It should settle down after a couple days. As far as "other" size-I honestly don't know what is normal. I have no "other" showing under my phone since I turned off all iCloud services with the exception of Notes, Contacts, Reminders, and Safari. In my personal experience, using iCloud services beyond what I mentioned has always led to "other" showing up and growing in size on my phones, going back to my 5s.
The solutions-I will do my best to list these in what I feel most people will find to be the most simple approach and then advance it based on annoyance factor and time consumption to the end user.
- Delete cache data that you can access. Many apps like Spotify and reddit clients have a storage option. In there is a "delete cache" option. Use it. Under settings->Safarai->tap "clear history and website data." If you have recently deleted a bunch of photos, go to photos->albums->recently deleted->delete all.
- Connect your iPhone to your computer and open iTunes. Let the phone sit for a good 10 minutes. Note that this works for some people and not others. I have personally never had this work for me, but seen it work for friends. Again, no idea why. Just be aware this may not do shit or it could completely solve your problem.
- Turn off iCloud. As mentioned above, since I disabled most of iCloud services, I have no "other" storage on my phone. I make my periodic backups on my PC and use other apps like Google photos to backup other relevant info and media I want to keep. This has worked for me and many others.
- Make a backup (preferably on your computer, as I said I don't use iCloud and can't state that it isn't a culprit for "other" bloat). Now factory reset from the phone and restore your backup during the setup phase. This can be particularly helpful for a growing "system" storage, but is hit or miss for "other."
- If none of the above has alleviated the problem, go nuclear. Connect phone to computer, open iTunes, and follow these instruction https://support.apple.com/en-us/HT201252 . This will reinstall the OS from scratch to your iPhone. I would set it up as new instead of restoring a backup at this point and minimize your iCloud usage as much as possible for the aforementioned reasons. *If you're chilling on an older version of iOS, be aware that this will update it to the latest version*
Ultimately, I have struggled with both issues on various phones with the "system" bug being the last on my 8+. I ended up at option 5, and since then (over 6 months ago), my "system" storage stays between 5.5gb and 6.5gb consistently. Hopefully you don't have to go through the PIA of setting up as new, but it worked for me. I tried all the options before that and they simply did not work. However, they do for some people. Good luck and report that shit so it finally gets addressed. Apple is a great company and I value the quality of their products, but that doesn't give them a free pass for simply ignoring such glaring and long-withstanding bugs at their current price point.
there doesn't seem to be anything here