Ok...I'm going to try to take this in chunks because there are a lot of pieces here.
110 acres and Building Lots for Founders
There is no question that Macdonald thought his ideal course could fit on something around 110 acres, as he clearly wrote that in his 1904 Founders Agreement. There is also no question that Macdonald was searching for slightly over 200 acres to fit both the golf course and planned building lots for the Founders.
Macdonald viewed this golf course estimate is something of a math problem. He had previously defined the necessary ideal yardages for his "Ideal Golf Course", coming in at around 6,100 yards, and extrapolated how much width he thought he needed for fairways and thus his estimate.
Specifically, CBM wrote that he would need approximately 110 acres for the golf course, 5 acres for the clubhouse and surrounds, and the remaining 90 acres would be used for 1.5 acre building lots for the Founders.
In fact, six years later, 3 days before the soft Opening Day Invitational Tournament in 1910, CBM wrote to Merion (who were considering a developer's offer of "100 acres or whatever would be required for the golf course"...after CBM's one-day June 1910 visit Merion believed they would need "nearly 120 acres") that;
The most difficult problem you have to contend with is to get in eighteen holes that will be first class in the acreage you propose buying. So far as we can judge, without a contour map before us, we are of the opinion that it can be done, provided you get a little more land near where you propose making your Club House. The opinion that a long course is always the best course has been exploded. A 6000 yd. course can be made really first class, and to my mind it is more desirable than a 6300 or a 6400 yd. course, particularly where the roll of the ball will not be long, because you cannot help with the soil you have on that property having heavy turf. Of course it would be very fast when the summer baked it well.
The following is my idea of a 6000 yard course:
One 130 yard hole
One 160 "
One 190 "
One 220 yard to 240 yard hole,
One 500 yard hole,
Six 300 to 340 yard holes,
Five 360 to 420 "
Two 440 to 480 "
Earlier in 1906, HJ Whigham reiterated Macdonald's plan in news articles to provide building lots for the Founders, calling it "especially ingenious".
So we KNOW that was the plan well into the year 1906. That is a fact.
However, for whatever reasons, we know two things changed once Macdonald's search brought him to the land near Shinnecock.
First, he made an offer on 120 acres of land closer to Shinnecock than the present property. Obviously, at this point and for this property, Macdonald was only focused on buying enough land for the golf course. Perhaps it was water locked and no more adjacent land was available? Perhaps he thought he was so ideally suited he was willing to give up on the Founders Lots and return their extra money? Perhaps he thought he could get such a screaming deal that he couldn't pass it up?
We don't know.
What we do know is that after the owners rejected that offer, his attention turned to the land where he finally built his course and we know that once again he was looking for more than 200 acres, which is what he secured in November 1906 and purchased in the spring of 1907 (although George Bahto's book says the official purchase occurred in November 1907, so I'm not sure about that discrepancy).
We also know that the course Macdonald built opened at around 6100 yards, but was already expandable to over 6300 yards by early 1912, and was about 6600 yards long (or over 500 yards longer) by the time Macdonald wrote his book. Today's course is 6,935 yards.
We also know that the course today doesn't take up over 200 acres. In earlier threads both David Moriarty and Jim Kennedy used Planimeters to estimate the acreage of today's course and came up with a range of about 165-180 acres.
I think it's likely that a few things influenced this change. First, I think that CBM's idea of using width to create alternate strategic options for weaker players around some of the hazards created a bigger golf course than perhaps he originally estimated. I also think he underestimated the number of acres that wouldn't be usable as they were either swampy or water covered, reducing his overall land.
But, he had clearly promised the Founders "something in return" for their investment and faith in him, and he did his best to address this in his 1912 letter to them under the heading "Surplus Land".
Obviously, the approximately 25-45 acres left over wasn't going to satisfy the original promise of 1.5 acre lots for 60 Founding members, so it is likely that some other financial recompense was made after the fact, and I'm pretty sure that CBM was able to justify this quite easily to them in terms of it being all for the betterment of the golf course, and of course he was correct in that judgment.
Max Behr, writing in 1915, described the reality of Surplus Land.
And in actually laying out the course (which really laid itself out to a large extent) no concession was made to economy in the use of land. Even so a considerable part of the 205 acres is not touched by the course and is available for other purposes. And there you have the solution of the whole business.