Bullying

Yesterday my family came over for an early Thanksgiving.  It's gotten to the point where my parents leave us on Thanksgiving and go to the beach while I go to visit my in-laws.
Point being...my father gave me the greatest compliment yesterday.  Then at the end of the night, my son took his shirt off b/c he had been playing hard and wanted to cool down.  My dad leans over to my son and says "Good God Boy, you've got three rolls of fat there."  My mom chimes in and says, "yeah, you need a bra."  I screamed, "Hey!  He's only 5.  What are you doing?  Leave him alone!"
Luckily my son didn't understand what was going on but I am heartbroken.  I also now understand a lot more about how and why I am who I have been.  I have spent my entire life overweight and I am very careful that my son gets proper nutrition, lots of exercise, and that I never comment on his body.  The boy has grown so much in the past 3 months.  He's gone from 38 to 48 inches in height, gained 10 lbs, and jumped from a size 5 pants to a size 7.  He's also been an eating machine and does tae kwon do for an hour twice a week.  I know that my son is not on the path to being overweight.  I am just so upset with my parents.  I laid in bed last night stewing and thinking.  I know I have to talk to them but I have to do it in a caring way and I am definitely not ready for that yet.  The ironic thing is that both parents told me about a bullying incident that had happened the day before that my dad witnessed between an older boy and my son.  The older boy was being mean and aggressive and my son didn't know how to react.  My dad was really upset that a kid could treat his grandson like that and then he turned right around and did it himself.  I'm sure he didn't even realize what he did b/c both of my parents have always talked that way to me.  I have to break the cycle here.