I wouldn't get too off-tracked arguing about body / arena scale...

Not all of Sigma's Big forms are huge, some of them a huge comparing to X. Kaiser Sigma for instance, is only 3-4 times X's size, which is not huge at all. I would say Nova Dragon is even bigger.

Anyway, I'll elaborate on Kaiser and Colossus.

Kaiser Sigma:
In the game, only X-buster can deal damage to it, other maverick weapons were totally blocked by his body armor.

He can fly for a shoot time, shoot laser, homing missile, and release missile pods to aid him. In this form, he will control the battlefield pretty well, due to all the projectiles flying around.

Colossus Sigma:
This form is HUGE. It also has the ability to float, and teleport. (If you have played DMC, then think Mundus. He's about that size.)

He has several attacks. Spinning Fireballs, Shooting Fireballs, Green Energy Blasts, HUGE laser beam, and his fist. All of the attacks will do immense damage, especially his giant fist.


There are several things I'd like to point out:
Kuja is a caster, and casting magic requires time and concentration.
Being a caster I think his resistance to physical attack will be rather low.
His Nova Dragon is not as small as Kuja, and it can be easily targeted by most of Sigma's attacks. The dragon won't last too long if it is to take too much damage (from tanking Kuja, in order for Kuja to cast).

While in huge forms Sigma will lack speed and mobility, he can endure a lot more damage dealt to him, also, his defense will be much higher.