Bellyempire:
Buy her the size she wants and then get a couple the size you know fit her and say it was a mistake. She will see how it fits
I actually think this method is more likely to be upsetting to someone that isn't yet comfortable with their body being larger and who is trying to lose weight, especially if they are already feeling sensitive about it.
The "Hey, I got you some things I would love to see you in because I think you'll look cute," and finding your boyfriend got your size right because they fit well, feel comfortable and do make you look cute is a really sweet experience.
Whereas, "Hey, I got you some cute new clothes. These are your usual size, but I accidentally got you some a couple of sizes bigger. Only the bigger ones fit? Well good thing I got them then," is likely to be a much more uncomfortable or embarrassing experience because it will feel like her change in size is being pointed out and highlighted.
I might be a bit biased in this situation because my mother used to buy me clothes she knew were too small for me to try motivate me to lose weight long before I was accepting of my body and desire to be a big gal. It felt cruel, because at the time I was trying to lose weight but just wasn't able to get to the size she felt was appropriate. I've obviously moved past the trauma she caused me, but it took nearly 20 years. That sort of thing sticks with you.
People that aren't yet comfortable with their gains need to be treated a little more carefully and gently than those of us that are into being fat. Small actions that point out or emphasise their gains can be extremely embarrassing and even traumatising.