2 part picture, Chocolate Twilight (Brown chocolate look with her mane done in mixed colours of a dark chocolate with white chocolate stripe, Her cutie mark can still be normal as with dyed on or perhaps she is wearing clothes that look like protective tinfoil, normal colours with a metallic sheen and small ripples you see on say a Kinder suprise wrapper) proportions can be large already. Have her eating an easter egg with a lit fuse for the first picture with the second picture of her larger in any region you want with the egg exploding inside her and increasing her chocolatey mass.

Or do you want something simple like Bunnyfied Fluttershy with a basket full of eggs with lit fuses?

Hmm, Blink's pretty much on the ball with the "egg-bomb" ideas, can't think of anything Easter-ish that would go any better.

Combining "pony", "easter" and "explosives" is harder than I thought. :I