To get you started turning puny thingiverse things into Beefy Beefcakes, here's a ready to use meshmixer part and OpenSCAD module for quick and easy Beefy Arms!
EDIT: added an .stl for additional Beefification
EDIT: by the way, these arms were generated with the excellent MakeHuman, an open source anatomically accurate human character generator. It's pretty awesome and also generates a working rig that you can load up in blender to pose.
I also used the Blender to OpenSCAD exporter to generate the .scad module
to add the meshmixer part to your library put it in your /Documents/Meshmixer/Library/Default folder
to use the beefy_arm.scad, put the file in your /Documents/OpenSCAD/libraries/ folder and then in a new file type