Prevost has had various size bay doors over the years and I cannot explain why they did what they did. What I think is correct in my thinking is the bay that is the third back is the one dedicated on your vintage to OTR, so it was sized only large enough for that when the coach is equipped with it. That left the maximum space for the remaining bays.
Keep in mind our coaches are biased toward seated coaches so what might be an unexplained feature on any coach could be as a result of something done for a seated coach.
Other factors I am certain influence dimensions. For example, a motorhome chassis was somewhat unique because it had more rear overhang than that of an entertainer and a seated coach. With only a part of the 45 foot additional 5 feet between the axles, bays got larger, but not as large as they would with all the 5 feet extra between the axles as is the case with the entertainer shells. Now the wheelbases are all standard which has a benefical impact on things such as bay doors which can be standardized and that allows a standardization of parts in inventory.