Barbs don't get pregnant. If it is bigger than the others, it may be full of eggs, full of food, or could have a disease. Is it eating well? Any way you can get a picture?
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry