What I wanna say is that every baby is different. My older boy, who's now 5, was also exclusively breastfed. He weight gain was much slower than his baby brother. He's like only 12 lbs. when he's 5.5 months old. So, it really depends.
My little boy, also exclusively breastfed, weighed 13.5 lbs when he was 2 months + 1 week. I don't know how much he weighed now.
My older girl, also exclusively breastfed, weighed that amount when she was 4 months. Now, she is only 22 lbs, and she is already 2 years and 8 months!