The main reason to use a smaller diameter wire is that it takes less heat for melting and hence the metal around the weld is not heated to a very great extent.
Mig welding thin sheet metal.
To weld sheet metal with solid wires use electrode positive ep or reverse polarity.
As a rule when mig welding sheet metal never use a body filler metal that is thicker than the base metal.
Forget the larger 1 8 inch tungsten electrode and use a smaller one.
Don t attempt to weld thin materials like sheet metal with flux cored wires which put more heat into the base metal.
For thin metal this isn t a problem but a continuous weld seam weld would be much more appropriate for steel over 1mm.
Tig welding sheet metal.
The danger with this technique is the individual beads of weld might not melt into each other fully resulting in a weak joint.