Steps to Reproduce:
1. Create a UI Widget
2. Add a Checkbox
3. Scale Checkbox: Details > Render Transform > Transform > Scale > Set X and Y to 3.5
4. Run Game From Editor (PIE or Standalone)
5. Hover mouse over various points in the checkbox while clicking
Result:
The mouse only fires an event in a small area of the scaled checkbox (top right).
It seems that the mouse target area did not scale with the boundaries of the checkbox.