Technical Explanation!

It depends on the application this is what's happening.

Straight argon is not desirable for MIG on carbon steel - not enough
'oomph',the welds are pretty but they sit on top and are more likely to
cold-cast.

Straight CO2 is very hot, hard to use on thin material, and plenty of
spatter.  (Flux core wire does this)

You need either 25% CO2 and 75% argon, or a tri-mix which is usually
around  80%  argon, 15% CO2 and  5% O2 (the mix varies by manufacturer
and purpose).  Another alternative is 98% argon with 2% O2.

Straight argon is needed for aluminum, and it's usable for  stainless
steel (better is the 2% O2 mix).

The bottom line is that for most hobby MIG welding 25% CO2 and 75%
argon, is the stuff you want.

So you weld thin material then more argon  - thick material more CO2.