26.7 MB is because it's scanned book with text layer on top. If it was retyped in, let's say, LaTeX, and outputted to PDF it wouldn't take more than one tenth of the current size. Remember that scanning the book has also archival purpose.
has _scanned_ pages with text layer on top of it. That's the reason why it's so big.
I applaud retyping effort, as it's always better to preserve real content than images of it (even if OCRed), but I cannot say that I'm happy about troff being used for this purpose. It can be a matter of taste, but I don't like the way formatting is done in troff/nroff. That's why I never use it directly (e.g. using ronn to convert markdown text to man page, etc.).
But I understand it's done that way to preserve "the creation process" too, which is also appreciated. And the book is about troff/nroff, so dogfooding is present. ;)
