D B Campbell The Roman Army in Detail: The Problem of the First Cohort

[Renatus]

This means the centurion’s tent had the same space as two tents, so must have been a larger tent. Hyginus has the centurion allocated to the same space as the century, but no mention of the tent being larger. However, he does mention the tents of the Praetorians as being larger, so I wonder why he did not make this known about the centurion’s tent.

There's no mystery about this. Two tents are not pitched in order to leave room for the centurion's tent. Even if this does not take up all the space, as Robert suggests, it nevertheless indicates that his tent is bigger than that occupied by a contubernium and this need not be emphasised. On the other hand, so far as the praetorians are concerned, they are allocated double space and this needs to be explained.

Just being curious, but why would a centurion's tent have to be larger than the other tents?

There are a variety of possible reasons. Obviously, there is the issue of prestige. Then, he might have to share with his optio (there does not seem to be any separate provision for him) and, if the century had a signifer, possibly with him as well. There might well have been a couple of slaves in there too. Consider the size of the centurion's quarters in a permanent fortress. They are considerably larger than the rooms occupied by the common soldiers.