If a mare can be called “Sleeping Mare” when they are sleeping, a stallion can be called “Sleeping Stallion” when they are sleeping, too.
 
Here, Shining Armor ends up having a dream, where he is huge and is sleeping near the Carousel Boutique, while his entire family watches him feel very sleepy and tired, thus the family are surprised that even he would be the “biggest” Sleeping Stallion of Equestria. When he gets up from his nap later on, he will soon realize that he must take his entire family somewhere in the woods, so that they can see how much he has improved.