Try using visual alignment, and have the title read like a sentence. You can try some tradeoffs on the axis labels (truncation and word choices).
I don't know your use case and some constraints, but if you have some flexibility, you can give more clarity to the diagram by a combination of more explicit labels, and truncation where necessary.
Axis labels and readability
Here you have an opportunity to both declare the purpose of the diagram, and use visual alignment, nomenclature and position to tie everything together.
Rather than have users tilt their head and readjust to view the axis labels, put it on top as a sentence:
In this case, you can use alignment of the title, and repeat the term 'probability' in the top and bottom label to match the heading above it.
I replaced small with low, as we associate things going from low to high, rather than small to high, i.e. 'there's a low probability of x occuring'.
Now you have the title just above the labels. You might need to make a tradeoff here in terms of truncation. I'm not sure how much flexibility you have in terms, but rather than repeat the term 'small' for both axis, I used small, med, large to show progression.
Depending on the size of your labels, you might be able to spell out Small, Medium and Large, leaving only the first truncated.