The cat’s weight is entirely dependent upon whether or not it is sitting on your lap or not.
If it digs in with its claws it will momentarily become weightless as it follows a ballistic arc after being tossed across the room.
Cats can increase or decrease their weight at will. A cat will turn its weight down to almost nothing when it floats up onto the kitchen counter where it knows it's not allowed to be, or it will turn its weight up to the equivalent of a Shetland pony when it stomps across you in the middle of the night.
They can also put out powerful sleep rays, especially when purring.