The localizer back-course will be more sensitive because you're closer to the ILS antennae. On an ILS, the loc antennae are located at the departure end of the landing runway. On a Loc/BC the same antennae is used. Since you're flying towards the departure end of the front course runway (where the antennae are located) you're closer to the antennae throughout the approach.
On a long runway, it stands to reason that the BC would be much more sensitive than the front course.
Hope this helps.