Not much lighter though. The sport band is the 2nd heaviest band after the link bracelet (yes, you read correct; it's heavier than the leather straps and Milanese loop).
Do you (or anyone else) know why the black sport band and white sport band are different weights? According to this article, (in 38mm size) the white is 47g and the black is 37g.
http://www.imore.com/which-apple-watch-steel-band-should-you-get