Plants that don't get enough sun will tend to get taller and more spindly - "reaching for the sun". I think thats true of just about all plants. My tomatoes get ridiculously tall too. With dahlias, obviously some are taller than others due to genetics, but with less sun there tends to be more space between the leaf nodes. I have seen pictures of dahlias with 6-8 leaf nodes within the first 10" of height, that never happens in my yard, even the sunniest spots.
My Lucky Ducky, which I have had for 4 years, is a foot shorter (at least) this year, and I am sure its because it is in a sunnier spot. I have always planted them in the shadiest area of my dahlia beds, but this year they moved to a different bed, and they are shorter and bushier and more floriferous.
In Alaska, while your season is short, your hours of sunlight are much longer, so I would expect shorter fuller plants, if they are taller this year it might be due to more water or fertilizer? is that possible?
I am in no way an expert about this stuff, but I am pretty sure I'm correct with this.
