I've been reading and watching videos about Mesoamerica a little lately, one point that was raised was that it was more difficult for people like the Aztecs to project power far from their core territories in part because almost everything had to be carried under human labour. It is said that this meant campaigns could only last a few months maximum and supply lines were very short in comparison to the old world, and that this might account for things like the relative looseness of things like the Aztec empire more as collection of vassal states giving tribute to the hegemonic Tenochtitlan than a proper empire you might see elsewhere despite it's relatively small size compared to, say, the Roman empire or something.
I'm just curious if this a particularly accurate analysis and what exactly the repercussions were around the lack of animal power which seems so omnipresent in the rest of the world. I've also heard that one of the things that made the Incas so far reaching in comparison was heavy use of Llamas, which would seem to imply that animals, or lack thereof, had a pretty big constraining effect on Mesoamerican cultures, but that might be beyond the scope of this question.
If the argument is that the lack of pack animals increased the amount of equipment a warrior had to carry or that it made logistical trains more complex, then yeah, thats a fair assumption, but if its arguing that the lack of pack animals limited Aztec's supply lines, or made them unable to wage war on a certain scale then that would be an incorrect assumption, several examples of Aztec military deployments serve as example, for example the military campaigns against the Zapotec and Mixtec kingdoms involved deployments of several hundreds of thousands of warriors (sources claim a combined Triple Alliance army of around 300k), Axayacatl in his ill fated war against the Tarascan Empire fielded an equally huge number of forces against his enemy, only to be defeated by a Tarascan ambush of his forces.
Even if we go with the safe route and assume this are exaggerations, we still have several examples of such massive forces, most sources for example cite the Siege of Tenochtitlan has having involved forces of hundreds of thousands on each side with the combined forces probably involving something like half-a-million warriors (this is remember, after an epidemic ravaged the region, and left their forces severly weakened), for easy comparison, battles in the Dutch Rebellion involved forces of 10-20 thousand soldiers on both sides most of the time.
The argument of the aztecs having ressuply difficulties also seems difficult as they were in the practice of establishing road systems and fortress towns precisely for this porpouse on top of the expected support each vassal nation was expected to provide, Im not well versed in Incan history, but I believe they relied on a similar system of fortresses and depots to keep quick supply through their territory, the lama is also not an incredible pack animal, better than nothing, and very well adapted to the region, but not exaclty something to be relied exclusively on.
Now, on the matter of the the vassal states as your source claims the matter is a little more complicated, Im going to have to explain a bunch of things, but the Aztec, the Mexica didnt organize the Triple Alliance like that just because, empires had been organized in Mesoamerica (and many other parts of the continent) like that as matter of practicality, Im going to present 3 factors, the historical, political, and economical.
First the historical, so from the beginning, as you know the Triple Alliance was an alliance (duh) of 3 Altepetls (cities), Tenochtitlan, Texcoco and Tlacopan, with a... lets call it a division of power between them (religious, military and judicial), now this is a continuation of previous Triple Alliances that had existed throughout the centuries... think the Chinese Divine Mandate and you're in the right track, the first real evidence we have of the structural organization of the Triple Alliance as a political entity is around 856C.E., give or take a few decades for calendar errors:
In that same year there began to be 3 places of tlatoani or teuctlatoque government; the tlatoani from Colhuacan as the head of the (tri) partite government. to his side he put 2 other tlatoque, first the tlatohuani of Tollan at his left, and second the tlatohuani of Otompan at his right... and between the 3 they decided what was to be done with war or any other important bussiness...
This practice wasnt exclusive to the Valley of Mexico as the Tarascan Empire initially chose to organize itself in a very similar way, and the Mayans seemed to have practiced a similar political structure in the form of their Leagues, though obviously in more than 3 pairs.
Next, is the political, now, the Triple Alliance system continued for several centuries, until, a few centuries before the foundation of Tenochtitlan the structure passed on to the Texcocans, Azcapotzalcans and Colhuacans (the Colhuacans continued to be a mayor mesoamerican power for centuries), of these the Azcapotzalcans, under Tezozomoc, and latter Maxtla began a process of centralization and imperialization of the Alliance taking direct control of much of the valley, now what followed was the awesome War against Azcapotzalco, long story short, they lost to the new Triple Alliance: Texcoco, Tenochtitlan and Tlacopan, the war however, left a stigma against imperial and centralized rule much like how roman kings left a stigma against them in Europe centuries prior, when Moctezuma II followed in Mextla's steps just before the arrival of Cortez it lead to widespread political instability and deep mistrust between the members of the Alliance, but thats another story.
Finally its the economical side of things, one thing you have to understand is that empires are expensive, very expensive, now if you live in Europe, have a huge ocean in the middle of it through which you can transport vast quantities of material, your economy relies on slave labor, and you have a need to invade, say, Alexandria because it provides much of the food that the empire needs, then yeah, direct rule and conquest makes a lot of sense.
But if on the other hand you live in a rough, expansive (It might seem small, but most of Mexico is mountain terrain, in practice its a lot bigger than you can see in a map), but very fertile place, with one of the most advanced systems of agriculture ever invented by humans, well you dont really have to worry about territorial gains, what you do have to worry about though, is commerce, particularly "luxury" items, as things like food is not as important, the flow of material and merchandise becomes the most important thing for your empire, why conquer a city?, if you can profit many times over by keeping its infrastructure and controlling its exports of luxury items, why enslave their population?, when you have an acestral system called teqio by which they will take care of roads, bridges and infrastructure once sowing seasons (all 3 of them) were over, the only thing you have to worry about then is to keep your military and political infrastructure, how?
In practice it really isnt that much different to a European or Asian kingdom of the time, loyalty was ensured through law, connections, honor, prestige and blood marriages, and when it didnt, force then ensured territorial stability, in here, well, the Aztecs were just the latest and arguably most succesful example of the region.