There are several factors that influence how much space debris a spacecraft will generate:
1. Surface area vs. density - If a spacecraft is large and has lots of floppy solar panels and things hanging off of it, it's much more likely to be involved in a collision, and if it is involved in a collision, it's more likely that it will generate secondary debris dangerous to other spacecraft.
2. Orbital lifetime - If a spacecraft is in orbit for longer, it's more likely to be involved in a collision.
Cubesats are usually very dense, with few or no deployable components. Cubesats are also typically deployed in a low orbit that has a short lifespan - less than 2 years or so.
There are of course exceptions to both of these rules, but the phonesats are not.
Though I generally agree that CubeSats are relatively safe / low junkyness.
In fact, most (US only maybe?) CubeSats have to demonstrate an orbital lifetime of less than 25 years to get a frequency allocated. (The FCC regulating space debris via frequency allocation is an interesting debate for another thread.)