Weight Gain Problem

It must be in our genes. My Mom told me that I never gained weight when I was the age of Kyla (until now actually). I still remember how me, my sisters and my Mom would exchange tips on how to gain weight. And the scenario's the same when my Mom meets her sisters.

Now, on to Kyla. The transition between breastmilk and formula has not been that fluid, me thinks. Kyla's weight should follow the same curve but visibly, it started to shift starting December to January - taking but a mere 200g/month for a normal 600g/month weight gain for her age. The major changes at that period are none other than - Kyla's switch to formula and me getting back to work. Poor lil girl.
She already underwent some examinations like x-ray and allergy test but those tests revealed nothing. The prescribed medicines didn't improve anything either. Her pediatrician started to worry that he prescribed Kyla a two-page diet to be followed religiously in the next three months. On the menu are some spoons of meat and fish, a scoop of butter here and there, 2 servings of yoghurts for each meal, illimited doses of fresh fruits and vegetables, boudoirs, etc.

Now Kyla, if you don't exceed that 200g weight gain mark on your 7th month visit, can we just consider that we have the same genes and that you could eat whatever you want, regardless of sugar and fat quantity in it? And all your life, all your friends would ask your secret on how to keep a svelte figure and you would answer exactly the same as Maman? - That is, you eat a lot to maintain your weight. Deal?