My dd aged 2.3 has a huge belly and always has done so much so that her pants struggle to stay up. The doc looked at me as if I'd gone completely mad when I asked him to check. She looks like she drinks about 12 pints of Guiness a day!
I think some children have 'toddler's tum'. My son had an impressive stomach for years. Usually as they grow, the tum disappears. I think his problem was worsened by wind too!! So long as they seem healthy, I wouldn't worry.