Saggage is a mixture of things. It's a tendon (I think) that's been stretched via the weight and skin stretching. Massaging, moisturizing, and exfoliating can help skin tighten. I can't say for sure how much it will work but it could help.
Making your chest smaller is mostly just generally body fat loss and using T. The issue with weight loss is that you'll lose weight everywhere else so they'll just look proportionally smaller which doesn't really look smaller at all. One idea would be to make other parts of you look bigger. Building up arms/shoulder/back can help. By strengthening your back and shoulders, you'll be able to stand upright better which can help make your chest look smaller.