To properly manage aggressive territorial expansionism, or "blobbing" as we prefer in the colloquial, there are a few steps that could be taken, I think. One, restricting consanguineous marriages, so that individuals with a common ancestor within three or four generations cannot marry. This is not only representative of history (Habsburg inbreeding is truly a much later development), but also limits the easy accumulation of titles in the family by marriage, and forces outmarriage and more interesting inheritance patterns. No more cousins marrying cousins marrying cousins until all titles become as one because they're the only available marriage partners around with the appropriate dignity to avoid a prestige hit.
Furthermore, removing certain flashpoints for conflict would likely go a long way, as well. Dual or multiple homage, for example. If you manage to maneuver, marry, or murder you way into the duchy of Aquitaine, if that merely results in you also being the duke of Aquitaine in vassalage to the king of France and now paying taxes for it rather than seizing it wholesale and incorporating it into your realm, well, that's going to be a much less blobby outcome, wouldn't you say? Furthermore, the defender in a war of total foreign subjugation (not, emphatically, a claim war - titleholders with a blood claim should always be much better regarded than outright usurpers) should be able to marshal rather greater enthusiasm among their subjects than the aggressor, unless the defender is totally despised by his subjects: see, the English in Scotland, Ireland, and Wales. Attrition should be high, as well, to reflect the losses suffered due to prolonged guerrilla warfare.
The cost of warfare is another thing that should be addressed. War is expensive, and kings should have other, constant drains on their treasury and expenses to consider. The bulk of revenue should be derived from the king's personal possessions and holdings, with feudal vassals contributing to the realm in a military rather than financial manner, and extraordinary measures being available to raise emergency funds at the risk of alienating one's feodality. Both the Lionheart and King John often had to resort to exceptional means to finance their ambitious expeditions, and in this they were hardly alone among medieval rulers ("
In 1188, for example, the pope authorized, and the princes collected, the so-called Saladin tithe, a direct tax of 10 percent imposed on all clerical and lay revenues so as to finance the Third Crusade").
In theory the duty of the noble vassal towards his lord was a purely personal one and to commute it for a money payment was a degradation of the whole feudal relation. The payment of money, especially if it were a fixed and regular payment, carried with it a certain ignoble idea against which, in the form of state taxation, the feudal spirit rebelled to the last. When the vassal agreed to pay something to his lord, he called it, not a tax, but an 'aid' (auxilium), and made it generally payable, not regularly, like the tax-bill of the citizen, but only upon certain occasions - a present, as it were, coming out of his goodwill and not from compulsion; e.g., whenever a fief was newly granted, when it changed its lord, and sometimes when it changed its vassal, it was from the beginning customary to acknowledge the investiture by a small gift to the lord, primarily as a symbol of the grant: then, as the institution grew and manners became more luxurious, the gift increased in value and was thought of as an actual price for the investiture, until finally, at the close of our period, it suffered the fate of all similar contributions and was changed into a definite money payment, still retaining, however, its early name of 'relief.'
The occasions for levying the aids were various but always, in theory, of an exceptional sort. The journey of a lord to the court of his suzerain, or to Rome, or to join a crusade, the knighting of his eldest son, the marriage of his eldest daughter and his ransom from imprisonment are among the most frequent of the feudal 'aids.' The right of the lord to be entertained and provisioned, together with all his following, was one of the most burdensome and, at the same time, most difficult to regulate. Its conversion into a money-tax was, perhaps for this reason, earlier than that of many other of the feudal contributions.
Entities that start out large, like the Byzantine Empire, should be realistically plagued with internal stability problems, with the Byzantines suffering frequent revolts by ambitious generals, impostors, and revolts by Serbian and Bulgarian vassals. The Holy Roman Empire is very much its own entity, with many princes independent in all but title, and the Kaiser should be a very long way indeed from being in a position to command the entire empire's resources. The Iberian kingdoms should start out as suzerains to their Muslim neighbors, and be more inclined to collect tribute from them as they did historically, and to meddle in their internal politics, which would go a long way toward slowing the rate of the Reconquista.
Character traits are another point of consideration. Perhaps not all rulers are created equal. Perhaps an indolent lump of a king, utterly lacking in ambition, content to drown himself in women and wine, may not in fact have access to a kingdom-spanning invasion
casus belli, because it would not be in character for such a man to embark on such an exceptional course of action. Most great expansions, like those initiated by Genghis Khan, Tamerlane, Alp Arslan, or the Komnenian restoration, were not initiated by men most comparable in manner of living to the three-toed sloth. Such men would be much more likely to leave the expansion of their realm to ambitious vassals than to embark on campaign themselves.
Which brings me to another example. Henry II, King of England, first set foot on Irish soil October 18, 1171. The first English monarch to set foot in Ireland, despite landing at the head of a substantial military force (an army of nearly 5,000 men), Henry's primary focus was more to organize the territorial gains made by his vassals at their own initiative and to receive the submission of the Irish chieftains rather than to wage a war of total conquest for his own purposes. And, soon enough after, on April 17, 1172, he set sail to return to England, not long after being embroiled in a civil war against his wife, Eleanor of Aquitaine, and three sons. The English occupation and conquest of Ireland was a long and drawn-out affair, and as seen here, hardly one quickly or easily settled even by an ambitious king with an armed force behind him, and internal politics often conspire to limit foreign ambitions.