Unfortunately, this happens with most of the phones, because if a manufacturer says a phone has 8 Gb of storage, in reality you can use 4-5 and we all know that they get filled fast.
I recommend using an application to clean temporary files or files that are big an unneeded. Use the Play Store and install CCLeaner then scan your phone, but be very careful because the app selects images or any other big files (OBB files, MP4, APK files, etc).