Image imported from derpibooru 
Original Description:
 
Sunset and her friends have just graduated from Canterlot High and for the graduating class the school has organised a cruise for them. Following some bad luck and Rainbow being paranoid about possible ‘bad magic’ ruining their graduation cruise the ship has become stranded at sea with a storm oncoming.
 
Rainbow is convinced that this has happened do to some magic related thing, so she steals a life boat and sails to a near by deserted island to search for answers. Twilight and Sunset discover this and tell the others to keep everyone calm on the cruise while they go hunt down Rainbow.
 
They find Rainbow stuck in quicksand as well as being attacked by some kind of plant monster. Sunset discovers that the quicksand is actually a portal back to Equestria and pushes Rainbow and Twilight deeper into the sand and through the portal to safety.
 
Arriving in a dark cavern Sunset instinctively lights up her horn only realising after she has done so that she has turned back into a unicorn. Sunset also discovers that Rainbow Dash and Twilight have also turned in to their Equestrian counterparts. Before Rainbow and Twilight can freak out about the change their tops rip open as their breasts expand to match the size of their Equestrian counterparts. Rainbow is excited about her new larger chest while Twilight is just trying to cover up.
 
Sunset who was still young when she fled through the mirror is surprised to feel a sensation in her own chest and they suddenly greatly increase in size causing her to topple over from the weight. Getting over from the shock quickly Sunset is exited to see that she has grown so much, that she could even be close to her mothers true size.
 
Well now they have to first find some clothes that fit and then probably go find Twilight to get some help (Also maybe Sunset can compare sizes with Twilight).