My instagram pics look grainy/blurry on app but not on website?

Ok so I have a new instagram account and before I upload new pics, they are always high-definition looking in my camera roll but whenever I upload them to the app, it always looks just a tiny bit blurry/grainy and it annoys me. BUT, one time I checked my account on the actual Instagram website and was shocked to see it was actually in high-definition and the original quality in my camera roll. Is there a reason this happens? And is there a way to fix it?

If this helps at the moment I'm using an old iPhone 4 because my iPhone 6 is broken and I've had the 4 ever since starting my account. I'm just so confused as to why my pictures are low qualty-ish on the Instagram app but in higher definition on the website.

Any help will do.

I have the same phone. The problem is not due to Instagram, but to the hardware. Older devices have more problems. It could be a screen resolution issue (very common with Apple products) or an issue with the optimisation.