Luna's power play

Luna never truly gave up on her plan to take control of Equestria, just changed how she wanted to do it. The biggest change is that she no longer wanted to outright banish or otherwise harm her sister. The time they had spent together after her defeat by the Elements of Harmony had shown her that Celestia truly cared for her and it's not her fault how ponies view the night, but unfortunately if Luna wanted to rule the kingdom as she wanted that meant she still had to take her out of power. This is how she had come to her newest plan, fattening her sister until she's too big to properly do her duties. In her mind, it would be the least egregious way to claim sole rule while making sure her sister wasn't harmed, plus she could be more subtle this time. Encourage her to take that extra slice of cake, what harm could it do? An extra large meal after a long day of work made sense right? All of this supplemented by simple spells used on Celestia's food to increase her appetite while retaining every bit of fat in her food means that Luna's plan would be going great as Celestia poofed out. A few lies to assure her that she wasn't getting fat, rigging her mirrors to show her still thin, cracking down on anypony trying to tell Celestia she's getting big meant Luna's usurpation was guaranteed.