Im not exactly sure what to give a plant for the stalk to thicken, or "get big". However, i have found that "roughing up" young plants promotes very healthy sturdy growth. I generally take hold of the bottom of the stalk a give it a nice little shake. Much like a strong gust of wind. Also you can give the branches a few good shakes a day and that will toughen them up as well. Just make sure your not and idiot and shake it so hard it brakes. Basically your just giving the plant some "light stress", or weathering it if you will. Ive gotten to the point that my plants dont need to be tied up even with massive colas. This works very well in my experience.