daisybee, I'm sorry for the general negativity towards your question. This is the place to come and ask questions and get educated!

I am not afraid to admit that I was very much disappointed (but maybe disappointed is the wrong word?) that my Lab mix only got to be 45 lbs, I too wanted a larger dog. But absolutely does not mean I love him any less because of his size!!

I found this chart for the growth rate of Goldens:

7 wks. - 8-10 lbs.
9 wks. - 10-14 lbs.
12 wks. - 15-17 lbs.
16 wks. - 25-26 lbs.
20 wks. - 32-43 lbs.
6 mos. - 40-50 lbs.
12 mos. - 65-70 for males and 55-60 for females
2 yrs. - 55-68 females and 65-80 males
source

Your guy is 5 months (so that's about 20 weeks right? If I can do math?) so it seems as if he's right on target, but on the lower end.

Your right though, he may just be the runt of the litter. I have a Jack Russell terrier whose siblings all reached 20lbs but she is just under 10, but she's always been the little one.

I had to go back and find pictures of Rocky, he is such a cute little guy! I'd love to see more pictures if you get the chance!