It could be similar to @Mike theory about how the gap between Goku's base form and Super Saiyan 4 form seems considerably http://dbzeta.net/viewtopic.php?f=9&t=145&p=1324#p1324
But in this case, the gap of Base Goku/Vegeta and their SSJB is not that huge like how BoG Base Goku to SSJG, it's...