Volumes on Brent - front month volume on Brent is 42K contracts; vs 22K on the second-month; 11K on third-month.
Volumes on WTI are higher - front month volume is 435K contracts; vs 250K, then 188K. Roughly a factor of 10.
Brent tends to trade at a premium to WTI.
This was not always the case. In 2010-2011 the US shale boom created a glut of crude oil (collected in Cushing) faster than the pipelines could move the crude out to the Gulf of Mexico. Additionally the US crude export ban prevented excess barrels leaving the country. The US export ban was lifted in Dec 2015.