Space usage issues!!

Hello,

I have an iPhone 5s 16gb and I've been trying to update the ios software but it keeps saying it requires 2.5gb of storage. The phone says I have used 12.7gb and I only have 195mb of space available, so surely I would have 3.3gb of space available? I've tried deleting all my apps and I've gone through messages and email messages and deleted anything I could to free up space.

I know poeple who have the same 16gb phone have more data,pictures,music,apps etc than I do on my phone yet they have more space available on their phone, how is that possible! I'm so confused!!! I don't want to keep deleting stuff off of my phone.

Any advice would be appreciated!

Thanks!