To start with, this only affect COLT rifles. All other brands use standard, Mil-Spec-sized "small" fire control pins.
In the beginning, when Colt first released a civilian, semi-auto version of the AR15 to the public in 1964, they changed the size and center of the pivot pin between the receiver halves from the standard .250" (approx) pin to an oversize pin. This was, presumably, to make it difficult to put M16 uppers onto civilian AR15 lowers. At the time, of course, Colt was the only AR manufacturer, and SP1s were the only civilain ARs, so this wasn't that big a deal.
In the mid 80s, Colt's patent on the AR15 design expired, and other companies were able to start making clones. These other manufacturers used all Mil-Spec-standard parts sizes, including the pivot pin, for their lowers, as the only uppers that were available in the beginning were surplus M16 uppers. Soon, conversion pins were made to adapt SP1 large-hole uppers onto small-hole lowers, and vice versa.
Fast-forward to around 1993. Talk of an impending ban on "assault weapons" was being talked about, and Colt was revamping it's line of AR15s. During this time, Colt switched back to the standard pivot pin size, under pressure from the AR market that now had many other vendors making ARs, all with standard-sized pivot pins. The market was demanding interchangability, and so Colt responded. However, Colt was still looking to make it more difficult for people to do illegal M16 conversions (the large pivot pin obviously didn't stop anything), so at this time, they changed the size of the fire-control pins. Standard Mil-Spec pins, that everyone including Colt had used up to that time, were .154" in diameter. The new Colt rifles used .170" pins. This made it difficult to use M16 parts in these lowers.
Since around 1994, all of Colt's non-machinegun lowers, including LEO rifles, have used the oversize .170" fire control pins and parts.