WOW...that’s so heartbreaking to hear
In terms of her happiness I wouldn't worry about her not growing any bigger but do make sure she gets regular annual check-ups and x-rays of her heart (which I'm sure you've already thought of), that way if she does develop any problems you can catch them early and get her on treatment to prevent her heart growing in size (or at the very least slow down the process).
Hopefully, it's only ever going to be a problem IF she develops heart disease which A: Will likely only develop later in life and B: Would no doubt shorten the life of any cat (with or without a deformed chest bone).
What I'm trying to say is don't worry about it too much, keep her fit and healthy on a quality food (which I'm sure you already do) and hopefully Heart Disease will never be an issue. I know Yahoo have specialist groups for certain cat illnesses, maybe it's worth having a look as they may be able to give you some good advice on the matter.
Also…have a look on the internet for vets in your area who specialise in Heart Disease. That way, IF it does ever develop you can get her registered there for treatment.
For now tho, give her lots of loving and cuddles, spoil her rotten and don’t let it upset you