I have the Lincoln Pro-Mig 135 (115 volt) welder. I've welded serveral things together with it. The thickest thing I've ever welded with it was a winch bumper for my Jeep Cherokee. The brackets were 3/8" mild steel,and the tubing I welded the brackets to was 1.5"X3"X3/16". I used .025 wire(also using the argon/CO2 blend),and took my time and burned it in really well. It held up great. So great,in fact,that once I was trying to pull a friend up an embankment that he couldn't make it up. I was in 4-low in reverse,winching him with an 8,000lb Warn at the same time. My rear wheels came off the ground about 6",then I popped a front u-joint ,and the welds held together fine.
Yep, it's good old ohms law in practice. The thinner wire has more resistance which makes it hotter with the same voltage and current applied. The 110 welder doesn't have enough juice to run the .030 wire fast and hot. Takes a 220 circuit to get the amps to melt the wire effectively. I have a Lincoln 110 and love it. Total package was about $150 less than the Miller and welds just as nice if not better.