Walt, you are exactly right, as the legs go the mating frequency diminishes and therefore fertility wains. Interesting though, body weight and frame size/shape does not effect the physiological ability for a male to mate. Meaning, that frame size/shape does not diminish the males ability to produce viable sperm cells. So when a bird gets excessively large and fertility seems to drop, it is almost certainly due to infrequent mating. Like I said before, the vast majority of fertility problems are due to infrequent mating and not other issues with the bird.
One more interesting thing. Not sure why I always seem to think of 'something else' to add, but I do so here goes. There is a correlation between male body shape/type and competed matings. Trying to eliminate breed or strain characteristics, males that have more of a submarine type posture, similar to Dorkings or Houdans, in that the breast seems to be more horizontal to the ground, tend to have a lower percentage of attempted matings actually completed. In commercial Turkeys as they selected and bred for birds with increased breast muscle mass the male birds' posture changed into more of a forward leaning position. As this happened fertility decreased. It didn't decrease for lack of attempted matings, it was for a lack of completed matings. We have seen certain strains of birds that have been selected for breast fleshing begin to have a breast or keel bone that was more horizontal to the ground. When this has happened the percent of completed or successful matings decreased but not attempted matings.
Kind of makes you wonder how Dorkings and Houdans and breed with this posture have survived over the years doesn't it? Having said that, my Houdans are very fertile while my Dorkings struggle with fertility at times. Too many problems sometimes......Grrrrrrrr
