There are different factors that may affect this.
The BoundingBoxFollower component gets the local shape of the Spine BoundingBoxAttachment.
Its resulting shape in Spine can be from the local shape, scaled, rotated and skewed by its parent bone or any of the bones higher in the hierarchy, as well deformation.
BoundingBoxFollower is either paired with the complex SkeletonUtility setup (follows via a chain of local values), or the simple BoneFollower setup (follows the world values of one bone).
Because of the way Unity exposes Transform properties, BoneFollower can only realistically apply local values to the Transform it's attached to. So it can only apply rotation, position, and follow local scale.
If your bone's world scale is a result of any of the parent bones being scaled down, that won't be reflected by BoneFollower.
In most Skeletons, you can scale bones for visual effect but you usually don't want to scale any bones that are involved in logic (ie, aiming, colliders).
In any case, Unity Technologies also recommends you don't do any scaling on colliders since the underlying system doesn't support dynamic scaling anyway, and applying scale results in wasteful rebuilds of the physics object.
tl;dr
Make sure all of the bones leading up to that BoundingBox have the default scale (1,1).
If that wasn't the problem, let us know. 🙂