I have one as well that is always out of control. It gets pretty tall, but the problem is not that, but the sprawling nature of it. I have it in a large pot next to my front porch, and every year, I have to cut it back away from the porch and sidewalk.

I think it's just the nature of it. Mine would probably get taller if not in the pot.

Lots more sun and less water will induce branching as well as help keep the size under control. Some these Thai cultivars just seem to have an odd growth form though, so you may be stuck with it.

I don't grow these, but just the regular Euph Miliis.

Can't you go back to the vendor who sold them to you w/ yr. questions?

Well this one is the only one that is HUGE,and it came from cuttings I got from someone as they cut them back. It didnt come from a vendor. The ones from vendors are much shorter and dont bloom as much. They are named varieties, this one is unnamed. This one blooms hot pink with red spots, and then changes colors, to pink and green, and red and green. It is just huge though. Guess I am going to cut the bare stems to the ground. They are in the ground in full sun and whatever the Fla. sun and heat can throw at it. So I cant move it.