No one said anything about going slowly. The idea is to move more quickly putting more "punch" into the metal to get it to fuse but in a smaller area. A larger welder can do this. Let me try to put it in laymans words. I'm no engineer...
But try to think of this scenario for a minute, and then I'll try to bring it together. Metal disappates heat right? You know that if you ever handled a frying pan. Think of which frying pan you prefer? Which one heats up faster? Which one cools down slower? Each metal conducts heat at a different rate. Cast iron, and stainless conduct it much slower and holds it longer. In welding that is something you have to think about in regards to how much amperage (heat) will be required to weld it. Now think of which burner you want to put the skillet on? Do you want to be there all day? Are you going to but a 10" cast iron skillet on a 4 inch burner set to medium low heat to fry chicken? No, you are going to get at least the 8" turn up the heat to high to get the oil up to temp as fast as you can. Can you fry chicken on a 4" burner with a 10 inch cast iron frying pan? Yes, if you like greasy chicken and it will cook slowly and you'll have to keep it on screaming red high just to keep up with the heat being drawn away by the cooling ability of the metal. And you'll have a hot spot and all sorts of issues as the chicken on the outside won't cook well...while the middle chicken is scorching. But what do you do with the 8" burner? You can turn it down to med high or even medium and it won't even be glowing good and it will still fry that chicken and it will be light, crispy and fast cooking if you do it right.
Now put that idea into welding. The larger mig is the larger burner. The smaller mig is the itty bitty 4" burner screaming on high. The metal is of course the skillet. A larger MIG likely has more inductance capability too, which is another issue which helps the puddle wet in better. The metal will draw away the heat from the weld at a constant rate. A larger MIG can come in with higher voltage and amperage and deposit a clean fast bead with a fluid puddle without having to lay down a ton of metal to build the heat because it can easily overcome the metal's ability to draw the heat away. Building extra heat is not always a good thing. It distorts and stresses metal. Just like when I was welding with the 140E. I had to go slower so the puddle would stay fluid enough to build enough heat to wash the metal in somewhat. Each pass the metal built heat, and the next weld went a little more fluid than the other one as the heat built up. But IF I was to spray arc it for instance, I am guessing I could have welded a single thin pass before the temperature on the edges of the plate reached half the temperature of the plate did with a single pass of the little 140E.
Not sure if this is making sense...but to sum it up...It has to do with the welder's ability to overcome the heat dissipation rate to an extent. The area right at the puddle will melt and stay fluid longer.
One of the other reasons you don't want to make a thicker puddle (and making a cold lapped weld which appears perfect on the surface but does not bond at the root) is that the puddle cooling rate can be too rapid especially with a smaller MIG. If the puddle does not stay fluid long enough, the silicates, other contaminates, oxides, and gas bubbles which are present in every weld get trapped, creating a porous weld. This effect is magnified if welding on rusty/painted metal, or making a second pass on top of a previous pass with signs of surface contamination like slag etc.
A larger MIG will keep the puddle fluid long enough for the "bad stuff" to float to the surface, but keep the metal from overheating at the same time.
Now with all that said, someone is going to disagree and tell me you can take down an angry elephant with a BB gun.
