In my opinion, I interpret "nói chung" as "generally speaking". At this point, we all know "nói" = "speak". As for the other half, "chung" = "together". I think "chung" is more commonly used by southerners (I'm totally basing this off of the dialect my family and I speak).
That said, I would translate this sentence as "Generally speaking, this museum is not big". Maybe the DL team thought "generally" was good enough to pass as "nói chung"? Just thought I'd add my two cents.