Thank you for expanding on the koi
I don't think Bass need a huge mouth-full every time, but I do think a couple of things have to be present...1, high forage numbers that increase frequency of encounters with little effort to ingest, and healthy, stress-free conditions.
I believe you can get close to an assumption, but to know the true growth, we need to know the density of the fish being consumed ....
The standard is to relate consumption in energy units to energy density. That is, what quantity of energy is consumed at different quantities of energy per unit area. Comparing for example a 4.5" BG to a 3" BG ... to have the same energy density there must be 4 times as many 3" BG. So it would be taboo to feed equal numbers of each. For example, it would be like feeding the same number of 1/4" pellets as 3/8" pellets. The 3/8" pellets weigh 3.75 times more and so it would skew how much they are fed.
I "think" every encounter would be a different measure, although an average could be established if fish are sampled often but we know this is a stress inducer.
I think it more than just an encounter and how much energy was expended for it. A predator is expending energy all day and all night. To be sure, not as much per unit time as when it lunges for prey. But the time between foraging forays can be extensive. Also a foraging session can extend for a considerable amount of time without success. A few weeks ago I observed a 16" LMB foraging. I was well above it looking down and all the fish were behaving as though I was not there. The LMB would try to position itself relative to a potential prey and then accelerate towards it. But every time the prey would react and skirt out of the way in a direction askew to the motion of the LMB. I will mention that the LMB displayed foraging activity throughout the entire time. Constantly moving and trying to get a good angle and proceeding to accelerate multiple times. All of the prey targeted, at least what I could see were in the 4" to 5" sizes best I could tell. So this falls a little above the 1/4 mark. I do think that 1/6 to 1/5 length fish are more easy for an LMB to consume. At the time there was a dearth in smaller prey. Presently we've already had a spawn but fish smaller 4" are not numerous and are being consumed. There seems to be plenty of >4" BG ... though to be sure ... I don't know the energy density of them but this much I do know. The 4" to 6" BG are by far the most numerous BG in the pond except for brief periods following spawns.
Regarding individual encounter metrics, this just seems cumbersome. The best way to do a test is over a specified period of time where the average is embedded (and apparent) in the final result. Most research I have read does it this way.
Though energy expended per energy consumed is important for the efficiency of conversion ... we cannot overlook the LMB problem of survival. If feeding on BG it must consume an average 1.3% of its body weight daily or it will shrivel and die. It must be successful in consuming prey. Going without food for a day results in a loss of body weight in excess of 1% during the growing season. In other words, there is a daily cost to failing to consume a maintenance ration each day. MOST of the energy cost is between the lunges and attacks. Larger BG, because LMB are less successful at consuming them, require more investment of time (which is also an increase in investment of energy). The same applies to prey that are too small. To consume the same daily ration requires more encounters and this means more investment of time and energy. It stands to reason, that the most frequently consumed prey sizes are the ones that provide the better balance of energy investment and energy consumed.
For LMB and BG at standard weight, a 1/4 length standard weight BG comprises 2.41% of a 1 lb standard weight LMB. So if an LMB can eat 2.41% of its body weight in BG every day, it can grow an astonishing amount. It can almost quadruple its weight going from 1 lb to 3.73 lbs at an FCR of 9.42 in only 180 days. This would be a very unusual growth rate of course so suffice it to say, even in a pond where 1/4 BG prey is abundant, they must be relatively difficult to consume. I can say this with confidence because the limit of LMB weight in the above referenced pond seems to be around 3 or 4 lbs. I've not seen or caught any LMB larger than 19" and yet 4" to 6" BG are the most abundant both in number and combined creel weight (despite my efforts to cull them). If an 18" fish could consume a 4.5" bluegill daily the growth would be astonishing and the LMB in this pond are not growing beyond 19". So what seems to be missing is sufficient spawning and survival to the range of sizes that are the most energy efficient.
Would what it consumes most be most optimum? If not, why would what it consumes most be less optimum?
The second part of your question -or answer to-is less optimum might be most abundant and more optimum might be harder to catch.
There may be an answer to that that we don't know...might be an "It depends" type deal until someone proves otherwise.
No matter what hypothesis I consider, I consider the null hypothesis and every counter argument I can to discredit it. But I even do that with the counter arguments (Maybe I'm bipolar?). That's all I was doing in my comment that you quoted. A counter argument can seem tenable until what it must mean is untenable in and of itself. The question of what are predators actually doing has been thoroughly demonstrated. Since they are survivors, they tell the story of what fish are doing to survive. I just find the thought that the survivors are doing something more when it is less advantageous for them to be a very unappetizing thought. I would like to think that what they do most frequently is that which is most rewarding.