Condensing is when the user (Di'angelo or Erasi) will make an object or character smaller by "condensing" it with their paws/hooves. To do this, they will put the thing between their paws/hooves and move them downward, causing the item or character to shrink (however, Erasi is able to condense something if it's in contact with her hooves). This is usually done to make something small enough to fit into hammerspace or an equally small area ( Sonja's suitcase) (Di'angelo) or to make enemies smaller, so they're easier to subdue (Erasi). Di'angelo also did this to make his tail (Clementine) smaller (when he making himself look like Domino).
Enlarging is done so the object or character can be made to normal size or larger (after being condensed). Erasi rarely enlarges anything unless it's going to help her. Di'angelo usually enlarges things after he condenses it, but in other cases, after he puts it in hammerspace and takes it back out again, it seems to be normal size again, meaning either his effect on things doesn't last long or hammerspace is able to resize it. In Di'angelo in the Big City, Di'angelo seems to condense and enlarge everyone within seconds in Sonja's suitcase.