Why opening excel 2007 on a server takes 100% CPU when excel 2003 does not?

When opening a large excel file – say 5mb in excel 2007 on the server, it takes nearly 100% of the CPU. Opening the same file in excel 2003 takes only approx 30% of the CPU. Is this a server or an excel issue? – happens also with PDF files so thinking it is a server issue?