Small bin mixed with larger material can have various causes.

Most direct reasons may not be reasonable is to set the size of mesh, thermal hopper opening size with vibrating screen related to the frequency, amplitude, angle of the screen. 3-6mm cartridge material should actually be 2.36mm-4.75mm size. Such mesh sizes is set to 6mm, to 7mm, less common, if the part is too large there will be larger than 4.75mm into the 3-6mm bin.

If the mesh size is reasonable in design, most probably should be 6mm sieve is broken in some places, thereby bringing greater aggregate into silos at the fine level.