This generally refers to the TPI, or Teeth Per Inch, in straight folding saws. "Large" means less teeth per inch, usually around 6 TPI. Medium usually works out to 7-9 TPI, and Fine has a TPI around 15-18. The less TPI, the more aggressive the cut, which usually means speed. A higher TPI makes finer cuts, not ragged or rough, but takes longer to make the cut.