The original output is very large but when gzipped it's surprisingly small. For example an app in my framework that gopherjs produces is like 1.2MB minified, but the gzipped one is just over 200kb, so no problem.
Download size isn't the only issue. Larger scripts are slower to parse and startup, slow down debugging tools, inhibit JS VM optimizations and use up more heap in general.
This may not matter on desktop intranets, but it will matter on the mobile web.
This may not matter on desktop intranets, but it will matter on the mobile web.