As an example, I opened an Excel spreadsheet of about 10,000 lines (less than a megabyte) in both Excel 2000 and the free Excel Viewer. The latter software intentionally lacks nearly all functionality. Even so, it uses more than 12 MB of memory on my machine while Excel 2000 requires just 3 MB to have the same file open. A larger spreadsheet (multiple sheets, some with well over 100,000 lines, at a total of about 29 megabytes) opened in Viewer leads to memory consumption of about 109 MB while good old Excel 2000...well, it can't open that one since it's a docx and can't even parse it with the Compatibility Pack.
The newer file format was a genuine improvement so I really can't complain about that. I'm also using a machine with 8 GB of memory. That isn't a phenomenally large amount and I can handle a bit of software bloat. I'd still rather use the old, streamlined software any day, until I consider the other feature introduced in late 20th century Office versions: