Depends on what your comparing them to.

They are dwarfed by most media companies etc. Github's website sees far less than 1% of Facebook's traffic for example. On the other hand they have almost 30 million users so they are not exactly tiny.

Regardless it helped them initially scale and attract users by shipping features and focusing on proving their business. Down the road as they attracted users they kept their rails core but started adding in other systems that were met their performance needs.

Rails allows companies to grow quickly and focus on attracting users and delivering business value at the sacrifice of a re-write or complexity of managing rails and performance issues down the road.

Isn’t Github on sinatra?