Thread:Lockhqu/@comment-1398428-20170830221914/@comment-477304-20170831000725

There are couple things to considerabout that:
 * 1) The Master Emerald changing dize may be due to the developers fitting it into cutscenes/making it easy for Knuckles to hold. The same does not need to be done with the Phantom Ruby.
 * 2) The Master Emerald is a different gem, the same rules may not apply to all gems in the Sonic world.
 * 3) Changing size is one thing, changing into a completely different shape is another.
 * 4) Sonic Adventure 2 came out 16 years ago, technological advancements allows for better consistently than with SA2 and the Master Emerald.

It's not impossible for it to be the Phantom Ruby, but with everything we know, it's very implausible at the moment. If Infinite is using the Phantom Ruby, he most likely has it concealed.