Are they the same dahlia type? I have several--ranging from a foot tall to four feet, and the taller types are definitely leggier.

It may require support if it's a taller type--tomato cages work well, or a stake if you can put it in without damaging the tuber (I usually drop stakes in at the same time as the tuber to avoid damage).

If it's the same type, it may be getting a little more shade than the sturdier one. You might be able to pinch it to encourage more bushiness and less legginess. There's tons of pinching instructions online.