Will updating my iPhone software use more space?

I currently run on 7.0.4 and I could be running 8.4. I missed a few updates because I only use my phone for music. I have every megabyte devoted to music and I don't really see the point in downloading more software if it is going to take up valuable space on my phone. Would updating my software actually use more space, or would the new updates cause the software to become more compact? Some people tell me that the new software is significantly smaller but I seriously doubt it.

Yes, the upgrade for IOS 7 to 8 is around 5gb.

Well there's the fact the new updates bring new features, make existing ones run better, etc. Updates lately have been getting smaller in overall space though I'm not sure of the difference between iOS 7 & 8.4 in this regard.

If you can already do everything you want to do with the phone unde 7.0.4, there's no reason to update it. Just leave it as it is.