Its a design issue because server storage while expensive, is in this case not realy the issue (the costs are generaly not that high over time, and housing does not require an excessive amount of data either).
Note that each item in a house might require like 3 coordinates (probaby 2 bytes because of a float), a plot number, a rotation value (a single integer would normaly be enough, but i guess float is easier for storing as it doesnt require any calculations afterward). Ownership is decided by the plot itself, but it might still require some info about the FC itself (which actualy does require a bit more data). Each object can also have a few internal values (chocobo appearances), and this might be the biggest chunk of data.
But lets say that each appartment requires 10MB (which i would consider a lot! its effectively 100kb per object while 1kb would be a lot already), a small house 20, medium 30 and large 40mb. Even if a ward would have 50 large houses, this would be 2GB of data. A server can easily handle 2GB on SSDs. And RAM can easily store values well above 100GB on servers.
Data realy is not the issue unless they use very poor storage.
Its more likely that the limit has to do with bandwidth, even if its just internaly, hardware has limits on transfer rates, and upgrading this is very expensive and often requires additional work to optimize it further. Not to mention that upgrades go with big stability risks.
