The other night, as my clan-mates and I were engaged in the Iron Banner, a certain combination of unfortunate and somewhat unanticipated circumstances prompted one of them to roar, "You've got to be fucking kidding me!" And then on another occasion, "You've got to be fucking shitting me!" I pause a moment to think about those outbursts as they indicate someone rationally making moves when suddenly gravity is reversed.
Horseshit is a term which I think applies to a specific instance of falsity. And in that regard it would be less of an insult to 'call horseshit' on a particular matter. If I am saying that some situation is full of shit, it is generally horseshit to which I am referring. Likewise, I think my gaming friend had a moment of outrage, and the surprise of "You've got to be shitting me" must be inferred to indicate horseshit.
If the same thing happened over and over, I'm sure he would have thrown down the controller and walked out of the room exclaiming "This is bullshit!" Bullshit thus in contrast requires that one recognize and implicate a continuous and purposeful production of shit. An average cow is double the weight of an average mare and the average cattle herd is generally much larger than the average herd of horses, although I cannot say my experience with these observations is expert. Nevertheless, it's useful to consider these facts with the understanding that the bulls will produce more shit.
It is also useful to note that horses are much taller than cattle, run faster and range further. Thus their noses are not as likely to detect their own shit, whereas bullshit is generally found fairly closely to the bull.