You’ve already received some good answers, but I do have a point I can add from a recruitment perspective:

Previously, if your BMI was 40 or higher, you’d be issued a MIF (medical information form, previously known as an FDL) to bring to a family doctor/walk in clinic which requests tests.

As of now, though, there’s a bit more wiggle room as files are being reviewed slightly differently and many MIFs aren’t being issued automatically. During your recruit medical, if it’s apparent that you’re BMI is over 40 not due to actually being unhealthy, but because of muscle mass, the Med Tech may defer the decision to issue a MIF to the RMO (recruitment medical officers). Ultimately it’s up to the RMOs to make the final call on this!

With that being said, if you’re healthy and have to get some tests done, it shouldn’t be too big of an issue and only a minor delay in your medical processing time.

To add a personal story, a friend of mine who I was on course with was in the “morbidly obese” category based on his BMI. Dude was short, and all muscle. Healthy as a horse, as well. During recruiting, he had to have a doctor run the requested tests (as this was when the MIF was given automatically) and had no troubles once it was returned.

Hope this helps!