While I would expect the object scale is the culprit, you seem to have checked that. Also check any parent objects or armature bones, an object with a parent or an armature will show a scale of 1.0 but will be altered by the scale of the parent. A lattice or mesh deform modifier can also alter the dimension of an object without altering the scale. I am not sure that adjustments inherited from any parent objects will export to unity, but modifier deforms can alter dimensions if modifiers are applied during export.
You should also check that the scale is not keyframed, a coloured background means it is keyed.

Some constraints can alter the dimension of an object without appearing to alter the scale.

Check that your scene scale is 1.0

Another possibility is the exporter settings, I expect you would be using FBX for unity -

If the scene and fbx scales are both 1.0
I would try exporting to unity with the units set to none, metric and also imperial, and see if you get the same size each time. If there is a variation from changing unit settings (allowing for possible imperial to metric conversion) then you should report it as a bug.