because

Well Taka, I don't have your advantage of the complete text, however I don't see any reason here to change my view that 'because' refers to 'sense of the world.'  The author, by the way, seems to contradict himself/herself by declaring that smaller animals do not have an experienceof gravity equivalent to ours, yet claims that we as humans cannot recognize how different the world appears to smaller creatures.  Also, the author seems to equate volume with weight.  A hot air balloon has a small surface to volume area, is bigger than a human, but has a totally different relation to gravity.  Or consider a whale.  How do we know what a whale's sense of gravity is?  

This really is a semantic question, and the referent of 'because' can only be decided on the basis of meaning.  I'll try to explain my meaning one more time.  Every creature's sense of the world is limited (imprisoned) by the particular range of sensations designed into their organs of perception, of which the sense of touch--responsible for our awareness of gravity, heat, cold, etc.-- is one.  Taste, smell, hearing, and sight complete the picture we have of the world.  Humans have a different hearing range, for instance, than dogs and our normal range of detecting sound gives us a sense of the world that must--by mere logic--be different from that of a dog.  (We can, of course, use devices to increase our range of hearing, and be enabled to imagine the world of sound more like the dog's experience.) 

One could say that we, and all living creatures, are prisoners of our bodies, and one could superficially interpret that as a simple matter of surface-to-volume-to-weight ratios.  It makes more sense to me to interpret "imprisonment" as a consciousness of the limits of sense-perception, something non-human animals apparently do not have.  Is there an equivalent to Emanuel Kant in the non-human world?

So I say it is not because of our size that we are prisoners.

Our size is one factor comprising our sense of the world.  If we were a different size we would have a different sense of the world, in the same way that if we were able to see clearly for a thousand miles , we would have a different sense of the world.  Humans are "imprisoned"--as all living creatures are--by the limited range of their sense-perceptions. Most likely, it is the human being's unique self-consciousness that can conceive of the body as a prison.

Am I making any sense?

EDIT:  I should add "kinetic abilities" to sense-perceptions.  All creatures have particular kinetic abilities and kinetic limitations, only part of which are effected by the external force of gravity.

So your conclusion, then, is that the author believes small things are not "ruled by the force of gravity"?  This cannot be.  Size has nothing to do with freedom from the ruling force of gravity. If being prisoners means to be ruled by the force of gravity, then all living beings are prisoners, and size--or any other body quality--has no relevance at all.

Taka wrote:

Well, it's just a matter of relativity. ... It sounds like insects really enjoy some sort of 'freedom', compared to us human.

Of course, it's relative.  Being relative means that the degree to which gravity rules is the same for all creatures.  The ability to resist the force of gravity might be interpreted as a type of freedom.  The fly has it in its wing functions and in the microscopic grip of its 'feet'--we could be said to have something equivalent in our capacity to, for instance, fly in airplanes.

How do we compare flies with humans on the basis of being imprisoned by size?

Galileo said, "If a mouse were as big as an elephant, it coudn't walk."  His point was not  "smaller is freer than larger," but that mechanisms and stuctures fit to a small scale, do not work the same way in larger formats.  It's not a matter of the freedom that the mouse has over the elephant.  Though a mouse cannot pull a log through the forest, I doubt that it wants to, and I doubt that the elephant wishes it could scurry about at the speed of a mouse.  Which is freer?  And compared to what?  Is a cat freer than a dog, is a mouse freer than a cat, is a flea freer than a mouse, is a virus freer than a flea?  This is a subject for Aesop, or Dr. Seuss.

Davkett wrote:

Of course, it's relative.  Being relative means that the degree to which gravity rules is the same for all creatures.  The ability to resist the force of gravity might be interpreted as a type of freedom.  The fly has it in its wing functions and in the microscopic grip of its 'feet'--we could be said to have something equivalent in our capacity to, for instance, fly in airplanes.