Nappy Issues

Does anyone else have trouble with nappies? My baby is 6 months and in size 4. Some days it's fine but others almost every nappy we put on gets leaked through. I've tried sizing up, same issue. 3's won't do up so the 4's aren't too big. We always make sure they're properly fitted round the legs but it leaks out of the back where there's no elastic. The only cause I can think of is that sometimes the volume of one wee is just too much for the nappy to absorb at once. Baby does have a tendency to hold it in (often not going at all overnight!) but I don't know what the answer is if that's the problem.