The difference between size 5 and size 6 diapers is mainly in the size and intended weight range as well as total dimensions and the absorbent capacity.
Size 5 diapers are slightly smaller than size 6 diapers and are a bit less absorbent than size 6 diapers.
Size 6 diapers are a bit bigger and a bit more absorbent.
Many kids who can wear a size 6 diaper, may also be able to wear a size 5 diaper without issues, but they may need to go up to size 6 diapers if they are having leaking issues or are heavy wetters.
Size 5 diapers generally fit toddlers who weigh between 27 pounds to 35 pounds.
Size 6 diapers are designed to fit older toddlers weighing over 35 pounds.
Size 6 diapers also feature a longer rise, wider waist wings and even larger leg cuffs to prevent red marks on bigger toddlers.
The size 6 diapers also contain more absorbent fluff and polymer material to handle the larger bladder capacity of older toddlers.
Many toddlers are potty trained before they reach a size 6.
Some toddlers even potty train and never wear past a size 4 diaper, although some may need size 5 diapers and rarely size 6 diapers.
Size 6 diapers can also fit most 5 year old kids and even some 8 year old kids.
My 8 year old son could fit into a size 6 diaper and he wore size 6 diapers for bed wetting and he was skinny.
He wore size 6 diapers from 5 years old to a little after 8, before moving to a size 7 and 8 diaper for bed wetting.
However my son potty trained in size 5 diapers and never needed size 6 during the day, but did so during the night.
But every kid and toddler and baby is built different and some need larger size diapers at different ages.
And some need smaller sized diapers at different ages.
My 2 year old nephew is skinny and wears a size 3 diaper just fine, but his brother who was 2 needed a size 4 diaper.
So it can vary and you can't just rely on the age and weight.