Right...two possible buildings...
The World Showplace is actually between Canada and the UK. I'm not sure that it is very visible from other areas within Epcot. It is a L-shaped building.
The building that looks really big(I'm not sure why...I'm sure it is an illusion but I've looked several times and it just seems so massive) seen behind and a bit to the right of Canada if looking from across the lagoon is the Soarin' building, which is actually behind the Imagination Pavilion as well.