Note that the repair of some things do not need to count the whole weight in order to repair the item. The damaged part of a wagon wheel, for instance, would likely be less than 4 lbs. in weight (we would not count the whole wagon). However, to strengthen an item requires that the whole item be adjusted (because any part of it could give and thus break).
Strengthening soft items such as cloth, leather, books or flexible woods will improve their saving throws by +1 against breakage or destruction. Strengthening hard items such as metals and inflexible woods will improve their saving throws by +2. Some items, such as paper, foods, glass, ceramics or artworks cannot be strengthened. Other items, such as musical instruments or magical items cannot be strengthened without destroying their inherent qualities.