
If it isn't worms (which is definitely a possibility), has he/she got into something they shouldn't and overeaten? I know whenever I've had pups that have managed to steal food (my youngest Golden for instance managed to break open the food cupboard and stuffed himself) their stomachs blew up the same and they did of course get terrible diarrhoea.
It can also be gases/wind due to having eaten something that disagreed with them.
It can also be normal, some pups are very fat. :)
The big question is how he/she is in herself, if there is any doubt whatsoever see the vet asap.