[PleasantlyFascinated]

It is a challenge indeed. But it's possible for sure. There's many men that somehow get huge breasts because of gyno (and not necessarily fat guys), and they really stay as masculine as it can be in general. The problem is there's no record of people doing it on purpose. I mean, there's so many on their journey who just keep it to themselves, not out of selfishness, but because society doesn't accept yet that men who like that exist. And to avoid being judged they keep it a secret. That and also that they don't feel like they fit, for example, in a group of crossdressers. We all know that typical way of acting some men have regarding avoiding being called gay or whatever. I'm sure that happens in the breast growing context too.

Tall order.
 
It depends a lot, of course, upon what are the masculine traits you are hoping to preserve.

From what I think I know on the subject, guys who appear very masculine by way of having enormous muscles, yet have breast growth, have gotten that way by accident.  They will in all probability have been using steroids and/or injecting excessive amounts of testosterone.  The testosterone soon gets aromatized into estrogens, and they wind up with an estrogen load comparable to that of a 21 year old girl.  Their testes have nothing to do in such an environment, which then go to sleep and shrink to the size of marbles.

For a guy to have big boobs without the use of exogenous testosterone, it is very probable that they are overweight, and/or have high levels of intra-abdominal fat around their internal organs.  Visceral fat is particularly good at aromatizing testosterone into estrogen (though subcutaneous fat does a pretty good job as well.)  Such a guy might have acceptable levels of DHT, but also has high levels of estrogen, which then leads to boob growth.

You’d have to determine what being masculine means for you, before arriving at the best technique for taking one isolated part of your body in the opposite direction.