Why Does My Sugar Wax Turn Out Different When Scaling Up The Volume?

I will throw out a suspicion, although I am not certain that it is the correct one.

What you are doing here is to create invert sugar with the lemon juice - if you didn't use that, you would end up with hard candy, not a malleable substance. The acid in the lemon is not an ingredient in the reaction, it is a catalyst. So I wouldn't wonder if your scaling up means that you get a higher proportion of the invert sugar, making the whole mass softer.

If my theory is correct, you should get something closer to the desired result by scaling up the sugar and water, but keeping the lemon juice the same as in the original recipe amount. "Something closer", but not exactly the same, because while the acid will be the correct amount without scaling, it might need longer time to create the same proportion of invert sugar in the increased mass - but you cannot really give it more time, since you are restricted by the maximum temperature reached by the sugar syrup. So in the end, if you want the exact same result, you will have to find out empirically exactly how much lemon juice you need. But as things stand, there is a good chance that using the unscaled amount will be a good starting point.

An alternative to rumtscho: My guess is you just have a difference in final temperature.

Lemon juice does help create invert sugar, and this helps create smooth candy that does not crystallize. Similar results are obtained by including corn syrup or honey for part of the sugar.

At 246.2, you wouldn't get a hard candy with or without the lemon juice. You will get a soft, or at least chewable candy anywhere under 265. Temperature has by far the greatest effect on your candy's consistency.

So, I think 2 things might have happened.

Thing 1: since you are using the same pot, that means you have a deeper pool of candy syrup. It's going to be hotter at the bottom and cooler at the top, and there could be other hot spots, so unless you're stirring your syrup, your temperature might not be totally accurate. If your thermometer is closer to the bottom of the pot, or in a hotspot, it might say 246.2 while the majority of the syrup is closer to 240, which would give you a softer overall consistency.

Similarly, the syrup continues to cook as long as it's in the pot. This residual heat would have a greater effect on a smaller amount of syrup. So that would also cause the larger batch to reach an ultimately lower temperature, and have a softer consistency.