Touying: How to use `utils.fit-to-height`?

How should am I supposed to use the utils.fit-to-height function?

I am unable to use it for images and even the example in the documentation, i.e. #utils.fit-to-height(1fr)[BIG] does not work quite right, as can be seen in the picture.

It was generated by the following code.

== Exemplary title
#utils.fit-to-height(1fr)[BIG]