I guess it would depend on which antacid you're taking. If it's calcium...it's known to calm the receptors in the gut (would have to check out exactly how)...and a calmer gut could mean more water is being absorbed from the stool, hence a more formed one.
Which one are you taking? Or are you taking an acid reducer such as Zantac?
I found a site just for you...verrrry interesting regarding the ph of the digestive tract. click throughout...very informative.