"What is the source for this 'tale'?"
Sorry guest - if it's good enough for Stith Thomson, it's good enough for me
The 'Ancient Indian' reference in Lee's 'Folk Tales of the World, are indicative that the tale, in one form or the other for a long time
WE know that Ballads like Hind Horn can be traced as far back as Homer's 'Odyssey' and Lord Gregory back to Chaucer

Can't see why drawn-out 'humour' should be disapproved of in any way Tattie, they take as long as they take
The humour of 'Shaggy Dog' stories are based on their length annd would be unfunny if shortened -
I think I agree with songs or stories being 'unnecessary' long, but that's a different issue (I always think that those who put an extra or repeat verse to 'Will Ye Go Tae Flanders' manages to miss the point of the song spectacularly)
Long books like 'Catch Twenty Two' need the time to develop their satire
One of the problems seems to be that attention span has reduced so much as to make many of the great creations of the past 'too long'
Jim