From Elann.com

Sizing for lace garments is very fluid, and can be changed to suit at the blocking stage, keeping in mind that blocking will add more length than width. Size can also be changed as described in the knit-to-fit instructions and designer’s notes below. If you knit-to-fit, simply increase or decrease your yarn requirements, according to the percentage you adjust the finished bust. e.g. if you were to make the bust 40” wide, rather than 48”, you would calculate that 40” is 17% smaller than 48” (40 divided by 48 = .83), so you would need 17% less yarn – i.e. 6 balls (7 x .83), rather than 7.