Haystacks was an immense man. From what I understand, he was just a hair under 7'0" and, at his heaviest, is said weighed close to 700 pounds. However, the problem Haystacks has against someone like Kurt Angle is that his size is much more of a hindrance than an advantage. A man like Haystacks has to expend a ton of energy just holding himself upright and moving about, so he's not exactly what one would call a great athlete. Physically impressive to look at in terms of sheer size, but that's about as far as it goes.
In terms of pure wrestling, simply put, nobody has as much credibility in professional wrestling as Kurt Angle. In the world of amateur wrestling, Angle is someone who pretty much did it all including winning an Olympic gold medal in 1996. We know that Angle's extremely tough as he won several of his most noted championships as an amateur while injured or having only recently recovered from severe injury.
During his peak years in pro wrestling, Angle weighed in around 240, so Haystacks has him outweighed by some 400+ pounds. As a result, it's obvious that Angle would probably be tossed around a bit and just generally overwhelmed by Haystacks' size. However, as I mentioned earlier, Haystacks' size advantage is so extraordinary that it creates more of a problem than anything else. If Angle concentrates on one of his leg, whether his knee or ankle, uses his superior speed and athleticism to stick and move, he's eventually going to do enough damage to where Haystacks' leg is too sore to do much. If Angle gets Haystacks on the mat with a weakened leg and continues to work on that leg as much as he can, the match is essentially over. Haystacks is so heavy and Angle is so skilled that he could inflict enough damage to where Haystacks simply isn't able to get back to his feet. I could see the ref being forced to call the match due to Haystacks being unable to continue or Angle eventually manages to make him tap out.
Haystacks is a huge man, but don't be fooled by that. Angle was the far superior athlete, the far superior wrestler and the far superior star.