I’ve heard this phrase tossed around in Terry Pratchett books and on The Young Ones. Problem is, there’s never a context. From what I gather, it’s a kind of rude exclamation… but what does it mean? Literally, that is. Er.
A big job is what one does on the toilet, or the thing that is in the toilet.
To be completely blunt it is a big crap.
It sounds best with a strong Scots accent, a-la Billy Connolly.
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Shadow of the Pigeon -
Weirdo of the Night
I’ve never heard him say big jobs. More likely to be “jobbie” than “job”.
It’s considered a somewhat peurile statement, mind you - roughly on the same level as bum and booger. 
I believe it may mean something else in Australia, but I’m not sure.
Luther Blisset is Everyman.
So Smile.
James Herriot wrote of Mrs Pumphery describing her pekeneses(sp) voidings as little jobs and big jobs, as an american would say #1 & #2.