Wow. Did he think that the surgery would negatively impact your psych wellbeing? Did he justify his reaction to it beyond saying he though there were psych issues in play? Ugh, he has no right to tell you not to get the surgery. I hope he does not try to step in and sabotage it for you.
Also, the only "psych" issues I can think of coming into play may be a question around transitioning, but that's totally up to you if that's part of the motivation. If that is his concern, he's clearly stuck in a time where this is seen as a concern. If he worries about how it may impact your relationship with men, he's equally stuck in the past... as a woman with a large chest and the associated issues, and not having a better reason from him as to why he objects, I can't help but agree that he's totally out of line. I probably would have sarcastically asked him if he had similar objections to implants...

Ugh. Sorry. I'm getting all sorts of pissed at him over this for you. Really though, did he give a more thorough explanation of his misgivings? I really can't think of any psych issue that would give me pause around a reduction unless you were constantly seeking out surgeries to "fix" every part of your body. It doesn't sound like that tough.