Particle collisions work currently by calculating the CENTER of a particle to the collision object.
So, no matter what the physical size of the particle, there may be interpenitration with other collision geometry.
Here is a tut that explains a "sortof" workaround:
https://www.geocities.com/jozvex/tuto...rparticle.html
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com