why does my others folder keep getting bigger and bigger and unable to delete it?

The 'Other' storage on your iPhone are files which do not fall into clearer category labels like applications, audio, movies, photos, etc. This is the space taken up by text messages, emails, documents, and etc. It is also where data from apps are stored; things like settings, it's web cache, etc. 

These files are not harmful to your device but if they accumulate too much space, over time it can blow up your storage space. All these things are backed up when you back-up the phone, which is why it's still there when you restore from a back-up. 

You'll never get rid of it completely, but you can get rid of big chunks of it. Here are some solutions you can follow:  

1. Make sure your phone and apps are synced in iTunes. 
2. If possible, make sure you have backed up or copied off any documents or data you might need, as you will lose it. A lot of apps allow you to email documents and data. 
3. On the phone, go to Settings > General > iPhone Storage. You should find a list of all the apps. The list is mainly 3rd party apps, not apps that come with the phone. 
4. On the right hand side, is a list of the total storage space the app is using including the app itself. The list is also handily listed in descending order of size. If you tap on on one of the apps on the list, it shows you how much space is being used by documents and data. You also have the option to delete the app. 
5. To delete the app, simply tap on Delete App. The app is deleted, and so is the data which is stored in the 'other' area. Repeat for other apps if necessary. 
6. To re-install the app(s), plug it in to your iTunes and re-sync your phone. The app(s) are re-installed with no data taken up in the 'Other' area. 

Hope it helps!