Okay, so this summer I'm planning on getting top surgery--I've decided I want a keyhole instead of a bilateral, because it's less invasive, there's less scarring, I like having nerves up there and so on.  The problem is, I gained a tremendous amount of weight when I was on T (I'm 5'1'' and I was 170 pounds at my fattest,) and I'm still in the process of losing it.  By now I'm at about 120 to 125 pounds...and I still have a C cup.  Before I gained all the weight, at about seventeen years old, same height and 110 pounds, I was nearly flat-chested.  I could bind with a handkerchief and pass...so I know I'm not naturally big.  The weight loss is worrying me, though, in that I might not be able to lose enough breast fat, or reach my target weight (100 lbs) fast enough.

How much weight do I have to lose in order to get a good surgery result, or get this surgery at all?  I couldn't cope with getting a bilateral mastectomy, and I wouldn't be happy with it, but I also hate having breasts.  I'm just worried my body's betrayed me yet again and gotten fatter overall in the exact wrong areas.

At 17 your breasts are still growing. You may not lose enough for keyhole. I hope for your sake that this is not the case for you.

Okay just a heads up - rapid weight loss could cause your breasts to sag which could be bad if you're getting keyhole. From what I understand, keyhole depends not only the size but on the amount of ptosis and elasticity.

You'd need to be a B-cup at the absolute maximum.  An A-cup is best.  In either case, you cannot have excessive skin sagging.

It's unfortunate that you gained weight 'up top' on T.  Given an overall weight gain, it would make sense to gain some up top (even cis-guys do).  However, it sounds like your chest grew, well, more than would be expected in a testosterone-dominated body.  Have you had blood work done to make sure that your estrogen levels didn't increase?  It can happen as a result of the T sometimes (too much T --> aromatizes to E).  I would have that checked out first, because if you have excessive estrogen, losing that chest weight will be difficult.  Another complication may be excessive skin--skin, once stretched out beyond a certain point, tends to have a hard time retracting to how it once was.  Best of luck getting your chest back to its original size, but try to keep an open mind about the double-incision technique.  The incision scars do take a long time to fade, but there are some excellent results (usually with more natural areola placement, compared to the peri technique), depending on the surgeon.