He won't have been doing many more work than the others. Their performances aren't very energetic it's just walking around in different directions always looks a bit of a mess to me. I mean look at Girls Aloud they have dance routines etc and yes they mime too

but that sort of dancing night on night would take it's toll on you, but not the 1D type although the travelling too.
I reckon he's quit due to these relationship rumours. To be fair they looked bit suspect those pictures - as they were not just fan pics were they - holding hands etc, arms round waist... i don't know if its just people obsessing or not but i think it's these rumours that are the reason for this 'stress' break not anything to do with workload/travelling. I do think he is wanting to do something solo wise too... it's been on the grapevine and what Cowell said was a clue as it seems he has been pushing for it.
At this point, even if you hate each other it makes sense to carry on due to the obvious financial gains!

I reckon he'll be back and he'll take back the whole i want to leave. Apparently Cheryl tried to quit Girls Aloud after the Ashley rumours for example - this sort of publicity can get to you and maybe that is what it is. If it is, management and the band will be out in full force trying to get him to change his mind.
I was more shocked that all 4 twitters from the other band members didn't have anything like Get well soon Zayn or etc... maybe there is something more to it? Liam wrote 'what a day' - that has sparked my suspicions that there is.