i want to mention that many of sigma's big forms are crammed into spaces that are barely big enough to hold his form and yet the "parts" of his body still move quickly..
i dont think that a small space would actually limit sigma but would actually be more troublesome for kuja if kuja is to dodge sigmas attacks.. it takes a lot of moving around to dodge most of sigmas attacks and the less space to move around the higher the chance that sigma could easily nail kuja with a nasty attack.. not to mention that to fly you need a lot of space to fly around and here we are talking about a cavern
kuja still has to "shoot" his spell attacks right? sigma uses a lot of obstacles and hindrance in all his attacks.. how well would kuja be able to adapt to that.. adapt and move quickly to a position where he can concentrate and aim a spell at sigma? without getting pushed or rammed into some spikes or lava?

i seriously doubt psychokinesis would work.. i have yet to see sigma get tossed around by anything and for the most part he seems near completely immovable in all his forms..
if sigma can jump through a fully charged plasma shot from X buster without it affecting his motion much id think its safe to say it would be very difficult to affect sigmas motion with anything.