you meant that literally? i was using 5 miles vs 5 feet as a comparison using figure of speech

the speeds at which flying is done at..
stopping and moving on the spot.. to the right places
in a small space its not easy to fly around at high speeds without smacking into the wall.. and thats the kind of reaction speed needed when dodging attacks

it would be impossible for a dragon.. which uses its large wings to fly.. to fly that accurately and easily within a small space

added also that sigmas parts and the assorted objects he will throw at kuja.. where will he dodge to if the space is enclosed and small? what happens when he is cornered between a wall of spikes and a hard place?

as far as blowing out the walls.. assuming that sigma could tear down the entire cave as well.. and escape in virus form.. so what?