In short, if you switch size, you still paid for a Ruby Cup purchase, which included a donation. If you want a refund, your purchase is cancelled and so is the donation cup. 

Explained a bit more in detail:

Ruby Cups are sold via our Buy One Give One model. So when you purchased your Ruby Cup, your purchase included a Ruby Cup donation to a girl in need as well. Our model is based on financial sustainability: this means that with your order you also covered the costs of the educational workshop and continued support given to the girls in the communities where we donate.

If you just switch size, you have still paid for your purchase and hence also the costs of the donation cup, so nothing changes in this case. 

However, if you want a refund, the costs of the donation cup are no longer covered and therefore your donation is canceled as well. Without financial sustainability, we would simply go out of business or be dependent on charity, so this is a core feature of our business and what allows us, together with you, to create social impact and improve lives continuously.