My 2 cents.
Big bulls are older bulls, Older bulls are smart bulls. Smart bulls are much more concerned with energy conservation having been through the rut a handful of times as compared to younger bulls who run into about any situation they can that involves other moose. Bulls, cows, whatever. They just want to get into the mix. The older bulls just want to have their cows and avoid drama if at all possible.
The big bulls don't want to fight if they don't have to because fighting takes time and energy and if they are cowed up, they run the risk of losing their cows.
Cow calling is much more effective on older bulls. Too many times to count I've watched bigger bulls move their cows away from other bulls and bull sounds, usually without making a noise or responding to call in any way. Once they are cowed up, I feel like this is their default behavior if the perceived challenger is far enough away that they are not an immediate threat. This all changes within 50-75 yards. At that point I think the big bulls perceive the threat as immediate and they have no choice but to confront the intruder. They will come in for the fight.
On the bull I took last week I watched this scenario play out pretty much to a T. On encounter #1 with my bull, I had been largely cow calling with some light raking that he most likely did hear. After one of my cow calls, I heard him start grunting about a quarter mile away. We closed the distance until I was pretty sure I would be able to see him and did some raking. He went totally silent. After about 15 minutes of no response, I spotted him pushing his cows away from us fast about 600 yards away downstream.
The next day we floated down to where we had last seen him and sat silent for an hour to see if he would pop out or give his position away. After no sign of him or the cows, I let out one soft, short cow call and he started ripping off grunts and came in hot, running and thrashing brush the whole way.
My interpretation of his behavior in these two encounters was that he was happy with his cows and had no interest in confronting another bull if he could avoid it. He was, however, very interested in rounding up other lone cows in his neighborhood if given the opportunity.
My partner's bull was a different scenario. He was by himself and I came at him with a combination of bull sounds and bothered cow calls. He came in fired up hoping to get in on the rut party he was missing out on and was looking for a fight with the bull that was pestering cows.
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Your calling strategy, in my mind at least, should vary based on the scenario and/or based on where the rut is at. As the rut progresses, the animals behavior and mindset changes pretty significantly and things that work earlier in the rut may not work or be counter productive later in the season.
When blind calling and/or searching for bulls, I do about 75% cow calling and 25% mild to moderate bull sounds and that has served me fairly well so far.
I think one of the biggest mistakes people make is over calling. As many others have stated, these critters senses are super sharp. From the first sound you make they have radar lock on your position. Over call, especially when they get close, and they will not only hear stuff they don't like, they won't see the things they are looking for (other moose) where you are calling from. This is why they "hang up" at relatively close distances and won't commit to engaging until they are certain of what they are dealing with.
My partner's bull came in hot to about 100 yards and hung up, staring us down for 10-15 minutes. I gave him some looks at my paddles through the thick brush. He wound up swimming the river to get downwind of us and survey the scene rather than keep coming in a straight line to where we were at. Crafty little fella.
Nothing works all of the time, and everything works some of the time...