weight does matter--for instance if you use a BottomLine (Superior #80 wt?) on the bottom you will be winding much more on the bobbin than a #30 Cotton thread. So while you will actually use the same amount of thread on top and bottom (the stitch is made between the layers which is why when folks don't use a bat they sometimes have more "pokies" showing--no middle to hide that stitch)the # of bobbins you'd wind would be less with a higher weight #!