they're just cool like that

When you post an emoji from one instance to a different one the emoji does not transfer to the new instance as an emoji but as an image file. This it is not it's emoji side.

So any Hexbear emoji, posted to lemmygrad, will be larger. The same would occured is someone from lemmygrad posted an emoji to a Hexbear thread.

At least this is how I understand it.