When I rode with Peter, that was probably 50-60 lbs ago, but I’ve never been (well, in my first year of college, after having my mouth wired shut for jaw surgery, maybe) anything close to svelte, and it wasn’t an issue at all. The only hesitation I’d have in riding with him now is that I simply do not have the fitness or balance to ride well to a fence at the moment. When I’ve been back at it for a while, I wouldn’t hesitate. And after a disastrous (for me) clinic with someone that is fairly popular as a clinician, I’m pretty picky about who I will ride with. With your experience and skill level, you will be fine, I would think!
Also, you may want to find Jon Holling’s YouTube video of Peter doing a critique/judging of his dressage test. It isn’t the same as a clinic, but it would give you an idea of his analytical style. 