why ethernet paload minimum size is 46 bytes?

Yes. The ethernet protocol was originally designed not collision free and to run on a shared medium. That means it is possible that two sender start to send about the same time and it comes to a collision. A sender first listens to the medium. If it does not receive anything it starts sending. The sender must listen while sending to notice whether somebody else started sending, too, or not. If it does then it must continue to send until the minimum packet length has been send out. The minimum packet length is designed to make sure that the transmission will reach all other nodes connected and all nodes are able to recognize the collision. Obviously it depends on the line speed and the length of the medium. Just think of a long cable and two senders at each end. If the would only send 5 bytes each at the same time they would never notice any collision because they receive the other transmission after they finished theirs. The minimum packet size is designed to make sure that the transmissions of one sender ends not before all other nodes connected. Then all know reliably of the collision and are able to retransmit later.

For further reading read

Obviously that does not matter so much in today's switched networks anymore... ;-)

Gerald