Looked at the website of the Nymex but couldn't find volume data for the mini, not even for the big car,anybody ?
Try this: on main page go to 'Markets' near the middle of the page, then choose the drop down 'Market Data' then the choice of 'Exchange wide data'. After the Agreement disclaimer, choose 'Fact Sheets' under Exchange wide data, and then choose the date you want. Up comes a pdf file with all mkts from that date. good viewin'!