As this question illustrates, it is possible to increase the font size in an emacs frame, not just a buffer.
Unfortunately, the solutions there also increase the size of the frame itself.
Consider the following use case. A few people are sitting side-by-side. Once in a while, two or more get together in front of one workstation to discuss something. It is then nice to increase the font size while keeping the size of the frame still, which makes it easy to resume individual work afterwards. How would you do this?