LST keeping them small? That's the idea! People use LST to keep the plants from getting too tall, to avoid them growing into lamps. Topping is so you get 2 colas instead of one, although not all strains respond well to topping.

And topping doesnt always help, sometimes you can top and get twise as much as one cola, other times you can get the same amount as you'd have got with one cola.

Depends on strain, and growing specs.