From the Super Mario Wiki
Giant Blue Shells are a rare variety of Gargantua Koopa Shells. They usually have unique abilities, in oppose to most normal Giant Shells, which usually just bounce around the stage.
In the Mario Strikers series, the Giant Blue Shells are items that can be collected by the players. When fired, it will shoot at the opposing team, freezing them when it hits them. However, unlike a normal Blue Shell, the Giant Blue Shell rolls over a player and keeps on going. This allows it to freeze other players, both the foe's team and the player's team. Another thing that separates the Giant Blue Shell with the normal Blue Shells, is that the giant versions only come in a group of one, whereas the normal shells tend come in groups of three.