The present study aimed to determine the impact of egg weight on embryonic development, breast muscle histology, and bone morphology of fast- and slow-growing embryos and chicks. For this purpose, total 192 eggs from fast-(Ross 308) (FG) and slow-growing (Hubbard JA) (SG) broiler breeders were used. Eggs were classified as medium (64 +/- 1 g, Megg) and heavy (72 +/- 1 g, Hegg) eggs. At 18 days of incubation, egg weight loss, embryo weight, embryo length, breast width and length, yolk sac weight, and bone morphometric parameters were measured and breast muscle histology were studied. The same measurements were repeated on the day of the hatch. Although yolk utilization of FG embryos from Hegg and Megg was similar, SG-Megg embryos were more efficient than SG-Hegg at utilizing of yolk nutrients at 18 days of the embryogenesis and the day of the hatch. On the day of hatch, Hegg chicks were heavier than Megg regardless of the strain. Embryos and chicks from FG-Hegg had more fiber area than those from SG-Hegg whereas embryos from Megg were not influenced by strain. Higher capillary number and capillary to fiber ratio was found in FG-Hegg embryos than those from FG-Megg. At hatch, the FG embryos had wider tibia and shank at 18 days and heavier tibia, femur, and shank compared to SG. Egg weight affected relative bones weight, being chicks from Megg heavier than those from Hegg. Lower tibia ash content was found in Hegg chicks. It is concluded that Hegg may be an advantage for a larger fiber area and capillary density. However, lighter femur, tibia, and shank of Hegg chicks with lower tibia ash content than those from Megg regardless of strain may indicate that heavier eggs could negatively affect bone properties of chicks.