Why does the instagram app on my iphone use 600mb of data?

The app size is about 21 mb but the data its storing is 600mb could it be saving photos?
Whatever the reason how can i prevent it?

One HD photo could be upto 5 MB. This means browsing instagram online is using up 5*(no. Of photos you are viewing)= lots of data

If you don't need space desperately then just leave it as it is.
caches help with performance.

iphone doesn't allow you to clear app cache like Android does but if you really want to then perhaps have a look at i-Funbox. No jailbreak required.