The benefits I see in double potting is that it's easier than doing a transplant and you get the benefits of the larger pot (larger root mass). The negative is that you end up ruining the pots since you cut the bottom out of it so you can't really reuse them.
The increase in size is due to going into 12/12, they hit their stretch period which is why they took off, I wouldn't solely attribute it to double potting which is just seems to me a different way of transplanting. And also you increased the lights from 350watt fluro to 600HPS, that's a big lumen difference. More photosynthesis = more growth.