Hi!
What happens here is the problem with a full random placement. Random means that there always will be a chance to leave some space empty regardless how far you crank up the value for generated objects (the probability will get smaller but never reaches zero). The benefit of this "easy" implementation is that it's very fast and stable even with many thousands of instances, a requirement for real-time usage.
Some workarounds for your problem:
In Mash you could start with an even distribution and use MASH_Random or MASH_Transform to add some small amount of offset on the Position of the objects. As alternative you could use other tools like Xgen or a manual approach for the placement of your objects.
Based on your first post i recommend to use particles, the attribute "Particle Size" in combination with "Self Collide" should give you the amount of control you want to prevent overlapping. MASH -> Utilities -> Connect nParticles to MASH. You need to run some frames of animation because self-collision is a simulated effect, but after this you can freeze the particle state and skip the simulation.
