Self Collision - Blender Cloth Sim shrinks mesh I have been searching for days and have not been able to find an answer to this question.
I have a simple model.  A thin cube with default collision physics applied and a plain 1x1 with default cloth physics applied.  I checked self collision, leaving this default too.
I get the below result only a few frames in.  It is like the mesh shrinks rapidly.  I have relaxed the stiffness. lowered the weight.   Also rendered with out self collision fine....I can not seem to work out why this is happening.
I have a sub divide on the plain with 6 steps...!  lowering decrease render time but doesn't help the shrinking.
Most likely your cloth vertices are self-colliding with their neighbours due to the Self Collision Distance being too large.
Firstly, when running a simulation always ensure the meshes are created at 'real world' scales - eg, for a bedsheet you might assume 2 metres a side (or 2 Blender Units). Ensure you Ctrl+A and 'Apply Scale' following any changes to scale - to ensure the simulation is using the correct scale.
Secondly, reduce the Self Collisions 'Distance' property. At Blender 2.83.2 this defaults to 0.015 metres (1.5cm). If your vertices are closer together you may want to reduce that value.