That's very sensible. But then you can't state that smaller images alone increased your conversion rate by 56%. Maybe your products are just more Winter-appropriate? If you can't control for confounding factors, you can't make a strong conclusion.
...On the other hand, in a contrived example, one could imagine smaller images could increase conversion rate by 1000% (if it cuts several minutes off the process; a website that takes 5 minutes to load will get no customers).
What I would like to see is an analysis of how much faster the shopping experience is post-compression. Perhaps for some of those edge cases (like your 250+ item shoppers)
