In Bleach, it is stated that the size of Shinigami's zanpakuto (sword) is based on their spiritual ability.
However, many strong Shinigami have smaller swords. Sui-Feng, for example, has a zanpakuto that resembles a wakizashi, which is a relatively small sword. Why does this happen?
Because a shinigami's sword isn't based on their spiritual ability alone.
It also depends on the amount of control they have over their power. Ichigo's sword was larger, but it was also brittle, out of focus if you may.
Ichigo's father stated that a huge zanpakuto means nothing, and that if they had no control, all captains would have skyscraper sized zanpakuto.
Since Captains have immense spiritual power, their zanpakuto also have potentially huge size and power. As mentioned in another answer, their ability to control their power results in a small sealed zanpakuto.
However, their Shikai (initial release) and Bankai (final release) can cause their zanpakuto size to increase, because they are no longer concealing its power. See, for example:
At greater stages of release, the form of the zanpakuto depends greatly on its specific abilities, but there is a potential for huge size.
Captain Class Shinigami and above have the ability to control the size of their zanpakuto, they all make them far smaller than their spiritual energy would make them to avoid carrying around a stupidly big sword.
