It used to be a normal size. Shimmering with magic, it had been beautiful, but just like any other soul gem. Slowly, though, it grew. Even Homura hadn't noticed until a few dozen timelines passed, but it was a horrifying discovery.
And then it began to expand exponentially.
In the second-to-last time Madoka ever contracted, her soul gem had appeared as a second moon, a glowing, pink mass of sheer destruction hurtling downwards towards Earth.
It didn't really matter, anyways.
By the time it hit, every sign of life had been wiped out by Kriemhild Gretchen.