How to creat good picture quality videos but smaller in size,(small in terms of kbs and mbs)?

I am a mac user, using imovies. while writing this question i am experimenting with imovies for accuracy of information. I imported a 285kb photo in time line and created a 4 sec movie without music. now cliked file>share>quicktime and in drop down menu,if email, web or web streaming, estimated size is 124kbs to 350kbs, if burn cd-rom size can be 498kb to 750kbs, but if full quality DV size is 18mbs. Now imagine when importing video from a tape or movie camera. So i am trying to find out a software or any other way, using which i can create good quality videos of longer length and smaller size. lot of youtube videos are very good quality, how are these made so good in small size.