Weight hasn't been an issue (yet?) for me in therapy. My therapist and I have pretty similar, average body types.

That said, your question seems to bring up a bigger question (for me) of how explicitly we talk about our therapists (and our relationships with them) when the discussion involves something personal - like weight (or sexual orientation, or politics, or parenting style, or whatever). I'm simultaneously terrified and fascinated about the idea of talking on a more "personal" (real?) level with my therapist about the nature of our relationship. But a lot of the things I might want to talk about involve my therapist's personal experiences, and I don't really feel comfortable asking her to go there, so to speak. She's been willing to disclose some things about herself over the years, but I'm every wary of what feel like invisible boundaries around what is and isn't off limits. I'd imagine that weight might fall into that category - it could be really important to your treatment, or somewhat irrelevant. You feel like it's influencing your ability to trust your therapist, and in that sense, it is important.

I highly, highly doubt your therapist will tell you to take a hike. But I get that it's a really scary thing to bring up. Let us know how it goes if you decide to discuss it?