I don't see any reason why Go would encourage large files. A Go package consists of every Go source file in a directory. You can use as many or as few files as you want per package.
We don't really have the "one class one file" beat into us like in java or c#. That means you gotta think a little to see how a package naturally splits up. Some people are better at that then others.
Yeah, I found it a strange assertion to make, so was wondering if it held any water. Sometimes different languages have different ways of working, which seem odd to novices entering the space from a different background, so I didn't want to assume that they were wrong.
Looks like it may be a case of coming across some less well written projects and making assumptions about the entire language, which is nice as Go seems fun :)