Hi @Ted,
Mosaic plots (which is the default visual you will get in Fit Y by X when you use a categorical X and categorical Y) do not divide the x-axis evenly unless there is an equal number of observations in each of the categories represented by the x-axis. You can see the documentation for mosaic plots here. There isn't a way to disable this scaling in Fit Y by X because it's a feature of the plot itself. And, it's a good thing! Because of that scaling, you can see the marginal distribution of your X-variable, which is quite useful to see in addition to the conditional distribution of Y for each X category.
If you absolutely need a plot that sizes the X-axis uniformly irrespective of the distribution of X, it won't be a mosaic plot. But, you can make such a plot in Graph > Graph Builder, by placing your Y variable in the Y Role, and your X variable not in the X-Role, but in the Group X role at the top. Then, toggle the mosaic plot at the top to make a mosaic-like plot.
I hope this helps,
@julian