Lifting light and doing slow controlled movements?

[PunchDrunk]

maybe your technique is better with the lighter weights, but u won't get muscle mass, they'll just cut u up. lots of protein and clean carbs + heavy weight = muscle mass

The ONLY thing that'll cut you up, is taking in less calories than you spend. Whether that is done by eating the same as always, and burning more calories by training (this can be ALL kinds of training), or eating less than normal, it's still a matter of calories in the end.
Therefore, saying that lighter weight will cut you up is false. It's a myth.

[DoctorKillJoy]

Yes, certainly. IF you know what you're doing, and get an individualized program, which is being done with an experienced trainer, who knows strength training and has a clear picture of what boxing is all about, physically. There's also a million ways for it to be detrimental to your boxing, but all training is like that. Too much sparring is bad for you. The wrong kind of sparring is bad for you. Too much running, the wrong kind of running, running at the wrong time etc., is bad for you.

What do you mean by running at the wrong time?

[PunchDrunk]

What do you mean by running at the wrong time?

Well, running a marathon the day before a fight, is definitely not the right time...

[DoctorKillJoy]

Well, running a marathon the day before a fight, is definitely not the right time...

gottcha. wasn't sure if you meant time of day or something.