"My (insert friend, cousin, father, brother, friends friend, etc, etc, etc, etc here) used to lift, and he was about your size, maybe 10-20lbs bigger"
Another classic gem from idiots out there. I ask, "wow! do you have a pic?!" One time my sisters bf pulled that with me. "My little cousin is 14 and catching up on you. 220lbs already, and he doesn't even lift weights." I see a pic months later, and it's from his pool party. 220lbs alright. Including the man titties, gut and triple chin.
That along with the "I know someone who lifts double that!" is always classic.
I find that your real friends will encourage you, even if they don't understand what it is that you are doing. If someone is bringing anything negative to your life, it is time to cut them loose.