How normal would it be for one of my breasts to be twice the size of the other?

Lift/implants/reduce: This comes down to an analysis of the differences: it may be simply making the larger breast smaller or the smaller breast larger. However it more likely involves a combination of both as well as analyzing the need for a lift or repositioning of the breast creases or nipples or parenchyma. Perfect symmetry is not a realistic goal and will set you up for disappointment. Goal: less asymmetry:).

While breast sizes..: Do vary, if one breast is literally twice the size, there are some things that a plastic surgeon can help you with to improve your symmetry, such as reducing the larger side or enhancing the smaller side. Best to have an in-person consultation, though.