Instagram Crunching everything!!!

Instagram most likely auto-scales your image to a pretty small size. So the original size may not matter.

But aside from that: Forget about ppi, it's moot and irrelevant, doesn't apply on screen. You can set it to 1 ppi or 10 000 ppi; doesn't matter.

It's just pixels. On the web, one image pixel is represented by one screen pixel. So if your screen is 2160 pixels wide, a 1080 pixel image will fill exactly half that screen width. If the screen is 1080 pixels wide, the image will fill all of it.

Yes, 1080 pixels is fairly low resolution. But it's traditionally been a comfortable web size because it fits on almost any screen. If you want more pixels, more resolution, you may need to scroll the image. It may not fit on all screens.

BTW it's pixels per inch; not dots per inch. It's a common confusion. But dpi applies to printers, not images. An image is often printed with higher dpi than ppi. They are independent.