There have been reports that this comet is very large. Actually, the heart of the comet, the nucleus, is obscured by the dust and gas that forms the head of the comet.
Nobody knows how large the nucleus is.
The brightness of the comet is not always directly related to the size of the nucleus. This is because typically only a fraction of the surface of a comet's nucleus is active. It is possible that this comet has a small nucleus with most of its surface emitting dust and gas. It is also possible that this comet has a large nucleus with only a small active region. However, this is just an educated guess.