Pending actual translation, it seems that this is the artist's headcanon about Sukuna's size as affected by the miracle_mallet. The small one Seija is holding is Sukuna's "normal" size, the larger one is after Sukuna uses the mallet to grow, and the tiny one we're all used to is what happens when (not quite so sure about this) the mallet's power runs out/is reversed.

Seija herself is provided for scale.

Thanks.