Also, to add to this, IMAX is 70mm which basically means you get can get more picture in a frame (just an image). Thriller was 35mm which means you could get less in a frame.
However, when you scale 35mm films up onto an IMAX screen, which is how most films are (at least near that resolution digitally), as not many films are actually shot in 70mm (to my knowledge, Christopher Nolan shoots mostly in this, but not for every frame in the movie). As ChrisC pointed out, it's still amazing to watch even if it's not shot in 70mm.
I will say that my explanation was fairly "old school." A lot of movies aren't actually shot in film nowadays, proper celluloid film, so you mainly just get films shot on high-resolution digital cameras and that is then scaled up to a massive screen (you have to watch out for pixelation though) or the aspect ratio is cut.