Because circle all have the same proportions. You can take any circle, and just evenly make it bigger or smaller to make it perfectly overlap with any other circle.

The ratios of shapes only ever change if their proportions change. That’s why every single square also always has the same ratio between it’s side and diagonal (√2).

And the ratio of a rectangles side to it’s diagonal will always be the same, regardless of size, as long as the aspect ratio is the same.