If souls do definitely exist, and therefore interact with our physical bodies in some manner. It could be that (even if they follow different rules) the portion that interacts with our physical form simply ends up being interpreted by physical perceptions, resulting in something like the soul feeling too 'big' for the body, even if size doesn't technically apply to a soul. It just merely translates this way as a form of recognition of the circumstance, even if it isn't the literal true circumstance.
Or it could literally be that the portion of the soul that interacts with the physical body is truly 'bigger' than the body, being necessarily related to physical factors itself to certain degrees.
Like say, if a soul must have a certain connection to the body to be linked (even if the rest of it is different), and this connection is 'location' which is being 'within' the body, then physical factors regarding location could come into play in minor amounts. If it's located somewhere, then it takes up 'space', and it'll take up that space in relation to where it is located.
In any case, I'm one such person who has a sense of my soul being 'bigger' than my body. Though in my case it's not so much in an actual size difference (when I envision the form I see of my true self, it isn't particularly bigger than this), it's more of a difference of presence/energy. Like, there is a field of presence that would normally extend far out past my so called 'body' but in the case of this form I am in now, it is held and turned within instead. The presence is my body in a sense, but at the same time not, if that can be understood. It makes me feel claustrophobic, like there are walls within my personal space, pressed tightly against me, making it hard to breathe. It is also rather painful in my case, not physically so, but it is like my energy is being held in a container too small and turning against itself, pressing outwards but not escaping much. It's difficult to explain why it is, but it is almost maddeningly painful to me at times. I do have to say though, that as much as it is possible that I have too much energy, it is also quite possible for me that my energy is rather instead not very compatible with this form (or potentially both to certain degrees). This said because I consider myself as related to Chaos, as well as a form of fire (though I do not really consider myself akin to a demon with the chaos aspect), which both seem somewhat difficult to relate in terms of this body.
So as for that, I don't really consider Souls to have size so much as some sort of 'presence' which viewed from a physical/human perspective comes to be felt as size even when it isn't really. Two different beings could be the same size, but give off a different presence (such as anger compared to happiness) that makes one appear bigger. Though this isn't to say spirits with big presence are angry <!-- s

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-->, just to give it some sort of example. This applies to various animals too, which sometimes enact ways of acting to seem more threatening, bigger than they really are.
The monster I am is the reflection of you.
I am the flame and chaos ever reborn, a devourer to renew, unbalanced to cultivate, and beast of the world to feast upon all.
Give of your blood and spirit, or flee in cowardice.