I believe the bobbin thread uses more than the top thread. I am currently quilting a quilt and using a different color on the top thread than the bobbin. I started out with new spools of thread for both colors and an empty bobbin to fill. I have already used up a spool of thread that I am filling bobbins with and am still using the initial top thread spool and there's quite a bit left on it. Either I was shorted on the amount of thread on the bobbin spool or the bobbin uses more thread.