Answer: This plant tends to be scraggly and loose the way you are describing it. It will be "leggier" the shadier the location and/or if it is receiving excess nitrogen fertilizer and growing overly lush as a result.
Hard pruning would stimulate additional stems or suckers to grow from the ground, so at this point you might want to leave it alone for several years to develop some thicker branches.
The older wood will be less likely to sprout from lower buds although the natural shape is to branch out low. As the plant ages, you may eventually be able to trim away some of the lower branches to give it a bit of a "trunk" effect to shape it like a multistemmed round topped tree. It will still bend down under the weight of the blooms -- they are just too heavy for it to stay upright.
Finally, there are different varieties of this plant and some will naturally have much larger blooms than others. It is possible the variety you have produces the smaller blooms and that even the best of care will not be able to increase their size.
I'm sorry you are having such a hard time with your shrub.
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