D.O.
Good Morning,
Whew, you poor little one! Soy formula is incredibly difficult to digest because of the large protein molecules. It takes a lot of digestive enzymes and gassiness that can cause a little breakdown on the lining of the stomach and intestines. It also creates such a firm stool that is hard to expell and that can cause little tears of the rectum. Probably the reason for less spit up it that the formula is thicker/heavier. Are you aware of formulas like GentleEase? The milk proteins have been partially manipulated so it is easier to digest. Look for a brand that is labeled with broken down milk proteins and see it that helps.
Good Luck
D.