What is the difference between "expand to" and "expand by"?

Expand to - whatever is "expanded to" is being reached. Example - the audience was expanded to include teenagers - so the teenage audience is being reached

expand by - how much something is going to be expanded, or how they are going to expand example - their cookie production has expanded by 2/3 since the beginning of the year! Or they are going to expand their market by giving out free samples.

I hope this helps. <(^_^)>