I have 4 objects produced in Blender in 4 separate files. 3 of them export with no problems the other one is enlarged slightly along the X and Y axes when exported to STL, the z axis measurement is not enlarged. The diameter of the object increases from 10.098mm to 10.744mm. I have the scene units scale set to 0.001 and the length to millimetres in the stl export menu. All of the files have the same settings, the Blender version is 2.91. The export scale is set to 1 and the scale units box is not ticked for all exports. A link to the problem file is below.
EDIT: The export to STL was not the problem, it was a mismatch between the location of the x and y vertices and the dimensions of the object. I don't know what caused the problem but the answer presented describes a workaround to correct the problem.