most sports bikes are designed with 1 person in mind too, so the rider will often be sat near the centre of the bike. When someone jumps on the back, the centre of gravity and weight is shifted backwards which will make the front become light.
As for hogs and tourers etc, they have often been designed with a pillion in mind, altho not all have the set up for it too (susspension etc)..
Having someone on the back just generally can make the bike feel a bit more sluggish than normal