Scotland was full of Labour safe seats, so they did make up a big contingent of the Labour party. But Labour has lost all talent in Scotland. The SNPs success has caused Scottish Labour to churn through leaders till they accepted their fate of 2nd place - currently 3rd.
That used to be the case. But currently GE predictions (yougov poll 4th April) show that SNP would only hold 19 parliamentary seats. Now, these polls can change a lot when you get to the actual election. But, it's the best info we have to predict the next election.
That poll has some interesting results though. If it turns out to be accurate, it's going to flip an English constituency that has been Tory since its inception in 1880 to Lib Dem. That's messing up on a huge scale for the Tory party.