Paramore is absolutely NOT bigger than any of those acts. Their popularity and Vampire Weekend's popularity may be slightly comparable, but Macklemore and Imagine Dragons are in a whole other league of popularity. Imagine Dragons is the biggest "rock" act right now and Macklemore is the biggest "rap" and pop act right now.
At least that's in terms of the United States. Sometimes comparing one's popularity in both countries is like stepping into the twilight zone.
As you say - popularity can differ massively between countries.
Paramore are certainly bigger than Imagine Dragons over here (England). Macklemore is very popular and I ASSUME he has had good chart success due to the amount of exposure he gets (don't know for sure) but I still think Paramore would be a bigger draw for an event like Reading / Leeds.
Vampire Weekend are popular but more so with the hip kids than the mainstream I think.
Please correct me (anyone) if I'm wrong - I don't really listen to the radio / charts / music video channels so I could be wrong, but that's from what I've seen.
Having a quick look on Wikipedia for album charts..
Both of Paramore's last 2 albums hit number 1
Imagine Dragons reached number 2.
Vampire Weekend's last 2 reached number 3.
Macklemore reached number 20?! (but Thrift Shop scored a number 1)
... which kind of backs that up!