Denote $D=\{x^2+y^2\le1\}\subset\mathbb R^2$ a disk.

Let $f:D\to\mathbb R$ be a function on it. I am interested in simple Morse functions, but I suspect the answer is the same for any function, or any smooth function, or any Morse function.

A contour of $f$ is a connected component of the preimage of a point, $f^{-1}(\mathrm{const})$.

The diameter of a set is the maximum distance between its points.

Can the diameter of each contour be made arbitrarily small by a suitable choice of $f$?

I suspect that no; namely, I suspect that at least one contour of $f$ will have diameter greater than, say, the radius of the disk, but I can't prove it.