The skin around the anus is very elastic and doesn't really matter how small it is....it's going to look especially small on a little baby. The problem is not where you think it is, but farther up in her intestines. Even diarrhea can be a sign of constipation, because there is blockage farther up and only more liquid substances can pass through. The grape juice will help with the constipation and gas. Gas also is a sign of constipation.
You can also try gripe water and prune juice and see if that is more effective.
If the baby is not breastfeeding, try switching to a formula that is lower in iron.