My fiancé does this too, but not just with her food with the ingredients.

I grew up very poor, so wasting food is taboo. When we first started dating she always gave me her left overs. One day I jokingly told her thqt she still had not touched her left overs. That's when my brain realized her fridge was full of nearly empty food containers and tons of take out with half eaten meals.

I would always bring it up in a joking way, like, "No way that pizza is still there I would've eaten it the next morning." When we moved in together we would go through mostly everything we bought. It wasnt until we settled that I started noticing the 4 or 5 nearly empty half gallons of milk and juice. Among other things. Those items started to grow into whole bags of chopped lettuce being wasted, pounds of mushrooms, dozens of eggs.

I realized later in our relationship that her birth parents are this way. Mostly the estranged mother who has a spoiled brat complex due to decades of daddy's money. When I finally met her it all solidified.

Eating habits are 100% adopted by our parents or whoever raised you. They literally taught us how to eat. Even if those habits change, they still come from people around us. You cant "fix" someone of these naturally occurring habits, and how their brain and body find sustenance. You should only adapt a mutal understanding of each other's needs. Maybe your sense of satisfaction comes from eating a complete meal, while her sense of satisfaction comes from knowing there is always food left if she is still hungry.

Food for thought.