If you've got the yen for travel, maybe visit an Asian country where she might be a "normal" size against the population.
I went to Vietnam and noticed the sizing difference for myself: suddenly I was oversized. I've also seen the opposite in other countries where sizes are much bigger and I'm on extra small sizings.
Vietnam is great for getting tailored clothing too - if you can avoid the worst of the tourist tailors.
Worth a try, if you can find a country that has a style she likes...
Before kids we traveled to Malaysia, Japan, Singapore. But since we are not really fashion-forward I don't think either of us cared to look at clothes there.
I did get a suit cheaply made and tailored in Malaysia back in college. One of the few pieces of clothing that still fits me perfectly now.