To follow up, the 10/10 model physically consumes about 7 inches of chain (that's 7 outer links and 7 inner links...almost starting on the 8th outer link). The 13/15 model physically consumes about 9 inches of chain.

Say, for example, a shift from the small ring to the large ring requires an additional 1.5 inches of chain (just to make up a number). That's 21% of the 10/10 model's capacity, and it would have to swing forward by some angular amount. However, 1.5 inches is only 16% of the 13/15 model's capacity. Therefore, it wouldn't have to swing as far forward as the 10/10 model would for the same shift. It's giving a smaller percentage of its total chain storage back to the drivetrain.

So while a shorter cage derailleur can't move its lower pulley forward and rearward as far as a longer cage derailleur does, with larger pulleys, it doesn't have to. Its angular movements are smaller for the same chain wrap requirements of the drivetrain during a shift. This is how a shorter cage derailleur can have a similar chain wrap capacity as a longer cage derailleur -- larger pulley size.