I also have 3 tobacco plants, each in a 3-1/2" pot, that have been growing, and been repeatedly pruned and topped since 2012. This is the same group from which I transplanted some to the garden in 2014. They grew into typical, normal size plants last summer. These remaining 3 plants will go into the garden for 2015.
What I can conclude is that a tobacco plant, like a traditional Bonsai, will self-limit its size to its available resources. They are clearly perennial (so long as they are protected from frost). If they are eventually liberated into the abundant soil of the garden, regardless of how long they have been stunted, they seem to reach their full potential.
Tiny seedlings within a sealed jar (a terrarium) are most at risk from algae and mold. If, purely by chance, they avoid these invaders, then they just sit there in the shade, contentedly recycling the jar's moisture. Those kept in 3-1/2" pots require watering and sunlight, and periodic pruning. None of these (sealed or potted) has been supplemented with fertilizer, beyond what was originally present in the soil mix.
Bob