I hear you. My son is almost 10 and weighs 50 lbs. he's about 50 inches tall. I'm 5'1" and his dad is 5'8" - our kids won't be big! I take him into the GP every 6 months or so to make sure he's growing along at the same curve and he always is. he's about 10th for weight and around 3rd for height, I think. He's just not a big kid
I'd probably do the tests since I worry there is something wrong, but only if the ped. mentioned it. I'd talk to the ped. and see.