Will losing weight after bilateral reconstruction adversely affect their appearance?

Your tissue expanders are there to create enough skin in your breasts so that implants can be placed. Weight loss should not interfere with this process but it's best to talk with your reconstruction surgeon about exactly how you want your breasts to look after you've achieved your weight loss goal.

After a mastectomy and reconstruction tissue expansion and breast implants is normally very little if any fat remaining in the breast area. Therefore it is unlikely that you're reconstructed breasts will show any appreciable decrease in size. If you lose significant weight overall your breast will have a proportionately larger in appearance. Therefore it is more likely that weight loss will enhance your final result.

Since you are having tissue expander / implant reconstruction there is very little fat involved.  I would not think that losing 3o pounds would have much of an overall effect. 

Best wishes on your journey. There are two issues you may need to consider, appearance and overall health. Long term health and chance of recurrence are helped by maintaining normal weight and regular exercise. The appearance of the reconstruction,as the other responders have said, probably will not be adversely affected by weight loss. I encourage you to pursue wieght loss, regular exercise, and healthy lifestyle. Good Lick