Mesh deforms unevenly

I have a figure wearing a pair of shorts. The shorts are a separate item. I’ve hit the limit for shape key deformation so I removed the shape keys and set up mesh deformation. It works better but… When the figure bends forward the right side of the shorts grows while the left side shrinks.

I’ve re-mirrored the shorts and the mesh deform for the shorts. I’ve re-checked the weight painting. I’ve double-checked everything is centered. The left side still shrinks while the right expands.

Any ideas on what I’ve missed? I’m using 2.79b on Fedora.

The deform mesh is weighted to every bone. I’m not going to track down which ones affect it incorrectly, there’s no need for most of them to affect the shorts at all.

Alternative way of controlling the shorts is to transfer the weights from the body to the shorts directly. Could try data transfer modifier to do it. The shorts should follow the body movement, and could then add additional deformation and correction on top.

I deleted all the “extra” bones. Weight painting looks okay. But still have an uneven mesh.