As someone who has been no contact with my parents for more than 7 years, I find this post absolutely shocking and extremely immature.
For what it's worth, I "blocked" my parents after nearly 4 decades of daily abuse, and hundreds upon hundreds of conversations explaining how they made me feel, the damage they had caused, and asking them to change how they treated me if they wanted to remain in my life.
The way your boyfriend has handled this, over one comment you were never meant to hear, is shockingly cold and downright cruel and makes him the bad guy here, not his parents.
Sneaking out of their house while they were in the garden without so much as a note or explanation is so immature that I don't even have words for it. You should both be ashamed of yourselves.
At least you want your boyfriend to tell his parents what they did wrong, and he should. He should not, however, "block" them over this.
You yourself admit that you are larger size. We all have our issues. My nose isn't as small as I wish it were, and if I walked in on someone saying something about it, yes, that would hurt, but I wouldn't never speak to them over it, especially if the comment was made as a passing fact, not something meant to be cruel or the brunt of a joke.
His parents were not making fun of you. You also overheard only part of a conversation--a private conversation in their own home. They did not seek you out to make a mean comment to your face. You yourself say that they have only ever been lovely to you.
They will certainly have PLENTY to say about you now!