All the Alicorns lined up without their enchanted bras, and poor little Fluttershy dwarfed by all of them even at 6 tall.
 
I spent a fair bit of time working out a breast shape that can scale to such large sizes without looking too balloon shaped, but still having the body and volume that looks appealing. Its a happy middle between realistic and Furry torpedo boob, or at least I hope, it looks good to me.
 
Alicorn Twilight is taller than pre-alicorn Twilight, and I probably could have had her in there for scale, but she was about 57.
 
Also, All Magic refers to when she was given all the magic from the three other Princesses. Definitely would need an enchanted bra to keep that from making her friends suspicious! Though it would have the added benefit of suppressing the power and making it easier to control, something canon Twilight didnt have help with.
 
Threw Nightmare Moon in there in the end because she was taller, and bigger in the chest than Celestia. Also more powerful (hence chest size), as Celestia needed the elements of harmony to deal with her.