You *can* say it that way but the extra words do not add any meaning or clarification, so it is not considered a *good* way to say it. A more natural and conversation construction would be:
- Considering [that] Korea's population is only 50 million, it was a smash hit.
Note that "smashing" is an adjective primarily (or only) used in the UK and it means "great, fantastic" but the idiomatic expression "smash hit" is used very often in the US:
- Adele's concert in London was simply smashing!
- Bruno Mars keep has written a lot of smash hits.