You can amend the soil to encourage a plant to produce abundant flowers and you can place the shrub in a location where the sunlight encourages it to produce lots of flower buds but you cannot make the plant produce "huge" flowers. That is a genetic trait. What did you have in mind?
You can try but a plant with fewer stems will just produce fewer buds, even if the stems are full of buds. I have not observed noticeable size increases in my mums when I have pruned lots of stems that broke off or that my dogs stomped on while trying to catch squirrels. The idea has some merit though. With camellias for example, you can increase flower size when you cut the number of flower buds (not the stems) or when you apply gibberellic acid on the camellia buds. But foliar applications of Gibberellic acid in soft-tissue of mums only increase mums' height by stretching the distance between leaves.