"I AM BACK BITCHES!!!! MUAHAHAHAHAHAHAAAA!!!!"

"Oh you've GOT to be kidding me!"

Nightmare Moon discovers to her annoyance that upon her return, she's not as... tall as she remembered... Or as powerful...
Only if somepony who decided that magically cloning her from pieces of her armour found in the Everfree castle, had not used average height pony DNA material as a base but alicorn DNA, then she'd be as tall and leggy as she used to be.
But really, how would you get your hooves on alicorn base material? It's not like you could easily sneak into Canterlot to get some hoof clippings or horn shavings from Celestia.
Normal pony DNA is a lot easier to get, just spit in a test tube, right? Who could predict that would mean that Nightmare Moon turned out not much taller than normal sized? I didn't do it on purpose, okay? I'm sorry, alright?