Aircraft create wake vortex from each wingtip.
The violence of this is dependent on the weight of the aircraft, so we provide more separation for a smaller aircraft following a larger one.
For example, at Heathrow, a 747 following a 747 would be 4 miles. A 737 following a 747 would be 5 miles. However, a 747 following behind a 737 does not require any vortex separation, because in that case the smaller aicraft is in front.