Another aspect to consider is that there is a tendency (especially when maps are prone to errors, as was often the case in pre-modern times) for lands further from the capital (and 'high civilization' in general) to be larger. Those that grant the lands typically did not have a realistic idea as to how much land was there - only a rough idea based on natural boundaries.
Also, there seems to be a relationship between how empty (of people) a land is and the size of its boundaries. Thus settlements along sea coasts tend to be smaller than settlements further inland, as sea coasts can often support more people than the drier interior.
On the other hand, once the density reaches a certain point, lands tend to get sub-divided into more and more partitions - the lot of them gaining an increased uniformity. Thus lands that have been densely settled for centuries show little difference in size regardless of whether they are on a coast, deep inland, far from the capital / population center, or directly next to it. Lands that have been settled for no more than a few centuries - and often less, and lands that are - for whatever reason - very low in density (irregardless of how long they have been settled) tend to vary in size due to natural resources that avail increased population growth and by their distance from the population center from which the land grants likely came or the divisions made / approved.
There are exceptions, of course, but in general the pattern tends to hold (at least prior to modern times). Now a days the high quality maps and remarkably fast transport system mean that this point is all but moot in some regards. There is no uncertainty amongst those dividing up lands as to how large the lands are. With GPS they can probably measure them down to the square centimeter - and possibly map out all their resources, as well!
In any case, I would recommend reading through the info at this link:
http://www.ac.wwu.edu/~stephan/Book/contents.html
Chapters 4 - 7 are particularly useful in this regard, although the map of the US in chapter one also helps a bit in understanding this.
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As to applying this to your situation:
Perhaps the lands are frontier lands, but recently settled. Maybe they were formerly held lands recently retaken after a century or more appart from the kingdom. Maybe some natural disaster or danger caused evacuation of the lands and only recently have they been resettled - decades later.
If the lands are more or less on the 'edge' of the kingdom - and if there is no kingdom along that border (either that priorly held the lands or that has often had trade or other forms of contact with the first kingdom) then the lands are likely to be notable in size. Not that those settling the area perhaps know this. Maps, as already stated, were quite poor in that time period.
It is possible that the PCs now have access to lands far larger than they realized they were receiving - or that the Crown (or Duke, or whomever granted them the lands) relized they were awarding. On the other hand, they are likely days away from other notable settlements, and they in all likelihood would have to be fully self-sufficient to survive there - let alone produce enough to fulfill their annual tithes, taxes, etc.
On the plus side, having access to so much more land - most of it untouched, makes it possible that they may find untapped wealth of various other sources currently unknown. Maybe some hills nearby have good sites for quarrying stone. Not only can they build their dwellings from it, perhaps some of the stone is of such quality (marble, etc) that it can be traded elsewhere (esp if a river or sea shore or other means of swifter travel is available). Perhaps the creatures in the area have pelts that are prized in the capital. Maybe there are salt or even gold desposits nearby. Perhaps the land is unusually fertile or the wood found in the forest is useful beyond mere planks - cedar, maple (syrup), etc. There may be vast sources of potential wealth if their lands are both large and relatively unexplored.
Now, if they live nearer the capital - the population density center - then their holdings are likely to be smaller, greatly reduced in most resources and sources of wealth (as these have been gathered for generations), etc. Likely they will be faced with more intrigue, as they will be right next to the site where most of the political action is occurring. Out in the frontier they may have to wait as long as a month before even hearing word of some major event (although word will likely arrive sooner - perhaps after a week or so - if the event requires something from them, such as metal for armor / weapons, their presense to swear fealty to the new lord and to attend the funeral of the former lord, their men for a gathering army, new tithes based on a new treaty perhaps, etc.
On the frontier their time will be spent making certain their settlement can survive, exploring the surrounding lands for new resources or to make new settlements as their current ones grow, perhaps dealing with wandering monsters or nomadic bands of goblins / orcs, etc. Near the population / political center, however, they will likely be facing less of those and more of intrigue with bordering estates, thieves and other crimes more common to urbanized areas, guild factions, scrounging for resources - perhaps forming new trade agreements, etc.
