My son also spit up more on powder. My friend who works in the baby dept at walmart called the two big formula makers (ross and the other one…can’t remember the name right now) and they said that the powdered formulas have different ingredients than the RTF (although they are comparable nutritionally), and the powdered formulas are usually thinner than the RTF’s.
Also, this is my theory, so I don’t know how true it is, but I noticed that when you mix powdered formula you have to shake it so hard to get it to mix properly that you end up with lots of air bubbles. I think those air bubbles can be a problem for some babies too.