Just wanted to throw this out... why does society seem to tell us that bigger is better when it comes to babies... and everything else of course! Your baby is gaining weight just fine and if you are wanting to exclusively breastfeed I don't see why you shouldn't. I'm personally glad my EBF son is on the leaner side- hopefully that will carry over into later in life.
I received a great comment the other day. Usually people comment on how small Henry is when they find out he's 7.5 months old- he's about 16.5 pounds (25% at 6 mo so not teeny but smaller). Then the other day this women came over to say hi, and asked how old he was. I said 7 months and she said "wow he's the same size as my friend's four month old! She must be feeding her too much!" How great is that?
Keep an eye on the wet and dirty diapers, get weight checked as needed and just enjoy breastfeeding!!!