Answer: The thickness of an EVA suit depends mostly on the design strategy to maintain constant volume with joint movement.
A pressure suit must maintain constant volume during movement. Volume change x pressure = work, so a decrease in volume means exertion for the astronaut. If they relax, the suit assumes the position of maximum volume and the astronaut assumes the posture of a scarecrow.
Three ways to reduce this work:
1) Reduce the pressure inside the suit.
The minimum practical operating suit pressure is 3.8 psi of pure oxygen. Any lower, and the oxygen is displaced in the lungs by the vapor pressure of water. This low pressure is a hassle if astronauts have previously acclimatized to “normal” 14psi O2/N2 air (as in the ISS). They need to pre-breath pure O2 to flush out N2 and thereby prevent the bends. If the suit can operate at 8.3psi, pre-breathing is unnecessary.
2) Use constant volume joints. One constant volume joint design is bellows with restraint chords. The restraint chord is necessary to prevent longitudinal expansion with pressure, like an accordion.
This joint works well for hinge joints like the elbow and knee. But it is restrictive for ball joints like hip, shoulder and thumb. Pressurized gloves usually use these joints, but design is a real challenge and the multiple restraint lines need to be “tuned” to get best effect.
This is the pressurized portion of an Apollo-era suit (without the outer layer) showing accordion joints at shoulder, elbows and knees.
Another constant volume joint design is sealed bearings. These allow rotation so they are particularly applicable for shoulders, hips and waist. If used in angled pairs, they can function as hinge joints as well.
This NASA AX-5 hard shell suit uses all bearing joints (except the gloves)
3) Reduce the air volume in the suit.
If there is no volume, then pressure x volume change = O. This is the strategy used in the BioSuit.
BioSuit by Professor Darva J. Newman
In the BioSuit, pressure against the skin is maintained by fabric elasticity rather than air. Nice idea, but there is a problem. Air will self-distribute itself so the pressure is the same everywhere in the suit. Fabric pressure is local.
Also, fabric pressure varies inversely with the local radius of curvature. If the radius is large (like the center of the back), no amount of fabric tension can exert pressure. If the radius is negative, like the armpit, the void must be stuffed with bolster or the suit will generate negative pressure.
A suit exerting variable pressure against different areas of the skin can cut off circulation in some areas but create spectacular hickies in others.
Many suit designs use a combination of these design approaches.
This NASA Mark III suit uses a combination of bellows and bearing joints along with a hard shell torso.