Go up to size 5 diapers for night time. My 2 year old wears size 5 during the day, but size 5 leaks at night so I put size 6 on him at night and those don't leak. Both my boys have been like this. They've always been small and low weight, however, the smaller diapers that fit didn't absorb enough for their bladder size. So, we had to go up a size larger for night diapers to accommodate for the size of their bladders.
my little girl is following in their steps. She's almost 8 months old now and while weight wise, she fits size 3, her bladder is too large for them and she leaks even in the day time. We just put her in size 4 diapers and while the tabs meet in the middle to hold the diaper on, she doesn't leak any more.