Ur weight is important as it along with ur height informs ur bmi and allows the anesthetist to work out how much medication to put u under. If ur bmi is too low then u may be unsuitable for surgery because ur at a higher risk of complications as its more risky on the body.
If u lose a considerable amount of weight that makes u lose a lot of volume to ur boobs then the implant size, placement and proceedure.
Prior to ur op ur weight will be recorded. If ur concerned by how much u have lost then arrange another consultation for ur surgeon to double check ur measurements and check over what was initally agreed. End of the day only ur surgeon has the capability of letting u know if there are gona be any changes
Communication, preparation and planning is the key to the outcome you want.