As a Historian I can answer this, the average height for the Romans during 200 - 300 B.C were around 5'5. I believe the figures are exaggerated, sure he was exceptionally tall but a realistic estimate would be 6'6 to 7'0 compared to the 8'0 the Romans claimed he was.
There's a number of reasons that they said he was tall, 1. Propaganda and symbolism, 2. Oral tradition inflation ("Very tall" -> "giant" -> "eight feet tall"), 3. He really was unusually large.
Some plausible factors could be that he came from Thracian or Gothic stock, Northern Balkan populations were often taller than Mediterranean Romans, he was raised in rural conditions, likely a high protein intake, and he became a professional soldier early, which improved growth outcomes.