I send a number of the small envelopes. They are great for things like a single puzzle book or a few blank cards or other small things like local souveniers. Keep them under 13 onces if you want them to go 1st class rate, otherwise it is more expensive to send because you have to pay the full pound rate for either parcel or priority mail for 13 - 16 onces.
They are really good to send when your contact knows others that don't get much mail, because he/she can just hand the whole package to someone and let them have the thrill of opening it! Like someone said, the contacts usually like to open the big boxes to share with the whole group and sometimes are more reluctant to hand the big ones to an individual.