The main difference is that Slimline picks are thinner by design to accommodate for European and Paracentric keyways. These keyways have less space to maneuver picks so having a much thinner pick helps with these types of locks. I do not know the exact difference in thickness. This is not to say that Slimline picks will not work on regular/non-European locks, they will. Personally, if I am not picking a lock where slimline picks are necessary I won't use them because I find the thicker regular picks are more durable. As far as i know there is no major difference in length or feedback that I can see.
Edit: ah you're asking about the thickness of the handles themselves. Cant help you there
