I recently downloaded XCode. The file size is 1.7 GB. In Activity Monitor, it indicated while downloading, that my upload speed was basically 50% of my download speed for the entire duration of the download. After the download was complete, the 'Data received' in Activity Monitor read 1.7 GB and the 'Date sent' read just under 1 GB (numbers are approximate). I wasn't doing anything else that would involve network usage. This isn't the first time that I've seen this pattern.
It seems to apply when using ethernet or Wi-Fi. If it makes a difference, I'm running Lion on a MBP, connecting through ethernet, and downloading with App Store. Below is a screen shot from Activity Monitor taken during a previous download.
What is being uploaded and why is it always approximately 50% of the download? Does this pattern apply to all data transmission?