Larger lines are thicker, requiring more space to hold the line. In addition larger reels are usually used for larger fish which may require more backing to play your fish. The reel designation also relates somewhat to the balance the rod and reel will give you as a combination, though this last statment is much up to the preferences (and imaginations) of reel designers and builders.

Another generality is that when using rods that are in the 3 wt. class you rarely see your backing and the drag is less important so it is not as critical to make the exact match.

That being said, it is always good to stay close to the rod designation when buying a reel.

jed