Smaller network packets vs bigger packets - confused

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http://serverfault.com – Some are saying that bigger packets are better to send then smaller. But in this app: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/message/messa... The lower the packet size is, the smaller amount of time it needs to reach the destination. So I don't understand why to prefer bigger size? Can you please explain it to me? Thank you (HowTos)