To the OP...the reason you don't need as high of an AI dose is because prop does not convert to estrgen at the same rate as longer esters, such as cyp and enth. The longer clearance time of long esters provides for more time for the testosterone molecule to interact with the aromatase enzyme, rwsulting in higher estrogen levels. Reduced estrogen conversion is also why prop leads to a lesser degree of water retention, in comparsion to longer esters.
There is also a common misconception regarding the different esters, which has permeated basically every board. It has to do with comparative potency of the various esters, per mg. We often hear that prop contains more testosterone, per mg, in comparison to longer esters. This is true...and because of this fact, it is often stated that shorter esters, such as prop, provide a greater muscle building effect per mg. This logic makes sense on the surface, but like many things in BB'ing, what seems to make sense does not always reflect the truth. Despite the greater amount of testosterone contained in shorter esters, per mg, research has repeatedly shown (dating all the way back to the 30's) that longer esters provide a greater anabolic punch, per mg. In every study, longer esters resulted in a greater increase in total protein synthesis.
This effect is due to the longer active life of the longer esters. This might at first appear contradictory to the personal experience of some, as some might say that they have experienced more rapid growth when using shorter esters, compared to longer esters. This may be true in the beginning, but in order to assess how much protein synthesis is occuring with a certain ester (and therefore potential growth), total protein synthesis must be measured over the entire length of the drug's active life. Because prop takes effect more rapidly, spilling a larger amount of testosterone into the blood right up front, protein synthesis will be greater initially. However, as protein synthesis continues to be measured over the entire active life of both esters, the longer ester will eventually catch up to and surpass the short ester, resulting in a greater amount of net protien synthesis.
With that said, many still prefer prop, as do I. The slightly greater degree of protein synthesis experienced with the longer ester is largely inconsequential, especially when using moderate-large doses of multiple drugs. More so, in my opinion, the reduced rate of aromatization experienced with prop and the resultant increase in hardness-dryness, makes prop a more cosmetically pleasing, as well as less side effect prone drug.