1. Chimneys at simplest.

At simplest, a chimney is just a vertical tube containing warm gases (A.) All things being equal, increasing the volume of warm gas by enlarging the diameter (B) or making the chimney taller (C) should cause it to develop a greater upward thrust. Much in the same way that a big hot air ballon will lift a heavier payload than a small one.
XXThe trouble is, all things seldom are equal with chimneys – and therein lies their complexity.