Like plumbing if you have **** in your pipes it will alter and disrupt your level of flow. In the case of networking there is a bunch of background noise and **** that get's transferred without much nuance however all of that adds up.

 

After reading for 5 minutes on Wikipedia a packet can vary in size depending on the network. Packets come in 8 bit increments and the header of the packet tells network the basic information about the packet size. Network data speeds just measure how many bits of data have moved in a given time frame, packet size doesn't factor in.