The impressive 40% reduction in file size is probably atypical. For comparison purposes I wanted to emphasize any differences between the two backrounds, so I had more backround showing than I normally would. I would normally crop this extra backround out of the pic, which would result in a less dramatic difference.
I believe the effect is still significant enough to consider when trying to achieve an optimum balance between smaller file size vs. acceptable picture quality. Of course other considerations often can't be ignored; for example, some subjects photograph much better with certain backrounds. In the end, it's the most EFFECTIVE photos we are after. The more techniques we can use to achieve that the better.