What do you mean bigger? The Matterhorn is about 200 feet higher.
In sheer mass, the Matterhorn is a relatively small jutted peak on a high base, Mt. Rainier is a volcano, the base of the mountain rises from pretty close to sea level to its full height and the mountain itself is close to 60 miles across. By mass it is one of the most massive, if not the most massive mountains in the world (memory fails me if there might be one more massive). If you look at each mountain from a similar vantage point (and I have) say 40 miles away, Mt. Rainier would fill the entire view, Matterhorn is a jutted peak of rock very, very small in comparison. It may be higher in altitude but it is in overall size much smaller, itd be like looking at Little Tahoma - the peak jutting out of the left side of Rainier, just highest in altitude.