As long as baby is gaining well, don't worry! If doctor doesn't raise concern abt baby's weight, it shld be fine. For many babies, formula may help to increase baby's weight faster, baby will feel fuller with formula milk, may need to be fed less often as compared to BM but of cz BM is better in terms of giving baby mom's antibodies tt FM doesn't.
Breast milk is best for baby especially small / preterm baby. I exclusively breastfed my 2kg newborn, she caught up fairly quickly and became chubby at 3 months. You can Google baby weight percentile chart or download this app called Child growth tracker. If the percentile increases steadily that's a good sign.
You can use the health booklet as a guide for the weight gain. Formula does help with weight gain but breast milk is always best for immunity of the baby. The docs or nurses at the jab or checks should flag out if they are worried.