I understand the issue with small displays and I took a short look at the code:
Currently the restriction is caused by the fact that the reciprocal of the zoom factor has to be integral for reasons. 1/0.75 is not.
Allowing zoom factors like 0.75 would require rewriting a lot of code and also slow down the preview at least for this zoom factors because we would have to resize from e.g. 100% to 75% then. For factors like 1/2, 1/3, 1/4 and so on the processing is much easier and faster.
As usual, pull requests are always welcome