In the Q&A on YT someone asked. They said it's on purpose to allow space for expansion and allow a gap at the top near the intake holes.
Idk if Troy tried it before adding more balls but I would imagine the heater can keep up even without the extra thermal mass from 2-3 extra balls.
If I feel...
They talked about this a little bit on their instagram.
Apparently a custom enclosure would drive the price up and it would pretty much be the same price/product as a herborizer.
They claim to be targeting heavy smokers who want to switch to vaping for cheap. The cord is slightly concerning...