Speed is a stat in all the Mario Kart games. Normally, speed depends on the size of the engine, and therefore the size of the kart. The bigger the engine/kart is, the faster it will go.
Usually, the better speed a kart has, the slower its acceleration is. If a kart is big and fast, when getting hit by an item, the kart will take a little bit longer to get back up to full speed.
Although this is normally common sense in the games, there are exceptions. Not always do karts need to be big in order to be fast. For example, the B Dasher (from Mario Kart DS and as a Retro Kart in Mario Kart 7) is not a big kart, and yet it is very fast.