Why are photos taking up so much memory on my iPhone?

I have a 16gb iphone 5 and under usage it says 11.5gb used and 1.7gb available? First of all this doesn't make sense and i was just wondering why it says this.

also, my sister has a 16gb iphone 4 and has 3000 photos and it only takes up 5gb but i have 1500 and it takes up 7.1gb! Why is this? Neither of us have any videos either!

and. I deleted All of my messages but it still says messages takes up 1.5gb? Why is this!
i'm running out of space and whenever i take over 50 photos i need to delete another 50 because i keep running out of space!

Photos taken by the iPhone 4 may be poor in quality than photos taken by iPhone 5. The memory size of a photos doesn't depends upon the number of photos but it depends upon resolution of photo. Photos taken by iPhone 5 are better in quality(resolution) so they are taking more memory space.

Photos from iphone takes more memory due to its high camera pixel (Mega pixel).it is generally less in other phone system.

You don't get all 16GB to use. Some is used to store iOS itself.

I never understood how people manage to have pictures take up that many GB or even close to a GB for that matter. I can't remember off hand, but I had/have > 2000 pics (mostly from shared Photo Streams) & it takes up less than a GB

As far as clearing up space, you could move the pics to a computer or upload them to a cloud storage box like Flickr, Google+, Facebook (all of which put them in private albums & leave it up to you to make them public), SkyDrive, Dropbox, etc

To further clear up space, download PhoneClean on your computer to clear up cache files.