Funnily enough I only ever used to drink Diet soft drinks. From what I've read though, they can cause water retention which was probably causing the belly!
I found that even by riding to and from work each day (roughly 6 miles) and playing football twice a week, that despite being fitter, I couldn't shift my belly at all. I recently upped my water intake to around 3 litres per day and gave up soft drinks and this seems to have had an impact.