Leonie - At three to six months of age, formula fed infants are consuming 66 ? 70% more protein than breastfed infants. This higher intake of protein is what may be a key reason for the different in weight.
There are also hormonal reasons, enzymes that differ and the simple reason that your average FF baby consumes more energy than a BF baby.
Hambreus, L. (1977). Proprietory milk versus human breastmilk in infant feeding: a critical appraisal from the nutritional point of view. Pediatric Clinics of North America, 24/1, 17-36
Heing, M.J., Nommsen, L.A., Peerson, J.M., Lonnerdal, B., & Deery, K.G. () Energy and protein intakes of breast fed and formula fed infants during the first year of life and their association with growth velocity: the DARLING study. American Journal of Clinical Nutrition, 58, 152 ? 161.
These two studies are the protein ones
Koletzko, B., von Kries, R., Closa, R., Escribano, J., Scaglioni, S., Giovannini, M., Beber, J., Demmelmair, H., Anton, B., Gruszfeld, D., Dobrzanska, A., Sengier, A., Langhendries, J.P., Rolland-Cachera, M.F., & Grote, V. (2009) Can infant feeding choices modulate later obesity risk? American Journal of Clinical Nutrition, 89/5, 1502 ? 1508.
Koletzko, B., von Kries, R., Closa, R., Escribano, J., Scaglioni, S., Giovannini, M., Beber, J., Demmelmair, H., Anton, B., Gruszfeld, D., Dobrzanska, A., Sengier, A., Langhendries, J.P., Rolland-Cachera, M.F., Grote, V., & European Childhood obesity trial study group. (2009). Lower protein in infant formula is associated with lower weight up to age 2 y: a randomised clinical trial. American Journal of Clinical Nutrition, 89/6, 1836 ? 45.
p.s. I am not saying these studies are perfect, just work in the area.